If you have been developing games, apps or industrial applications in Unity for a while, you have probably seen two scripting backends inside Player Settings.
Most developers know they are different. Few actually know why they are different. The real difference is not Mono or IL2CPP. The real difference is JIT (Just-In-Time) and AOT (Ahead-Of-Time) compilation. Once you understand these two concepts, choosing the right scripting backend becomes much easier.
Let's break everything down from the beginning.
Why Does Unity Need a Scripting Backend?
We write our games in C# because it is easy for humans to read. Computers don't understand C# source code directly. Before your code can run, the C# compiler converts it into Intermediate Language (IL) and packages it into managed assemblies such as DLLs.
Now Unity has a problem.
That IL still isn't the native machine instructions your CPU executes directly.
This is where the scripting backend comes in.
Think of the process roughly like this:
C# → IL → scripting backend → native machine code
The important part is what happens after the IL is produced.
Mono and IL2CPP take different approaches.
Mono primarily uses JIT compilation.
IL2CPP uses AOT compilation.
That one difference explains a lot of the behavior you see when developing and shipping Unity applications.
What Is JIT Compilation?
JIT means Just-In-Time.
The idea is simple.
Don't compile everything into native machine code before the application starts.
Instead, compile code when it is needed during execution.
With the Mono scripting backend, Unity produces managed assemblies containing IL. Mono's runtime can then compile the IL into native machine code at runtime.
The simplified flow looks like this:
C# → IL → Mono → native machine code
The important word here is runtime.
The conversion to native code happens while the application is running.
That has an obvious advantage during development.
You can iterate quickly.
You change some C# code, press Play, and Unity doesn't need to go through a massive native C++ compilation and linking process just to execute your managed code.
This is one reason Mono is very useful during development.
But JIT also creates a problem.
The target machine has to be allowed to generate executable code at runtime.
Not every platform allows that.
And this is where IL2CPP becomes much more important.
What Is AOT Compilation?
AOT means Ahead-Of-Time.
Instead of waiting until the application runs, code is compiled before the application is shipped.
IL2CPP takes the managed IL generated from your C# code and converts it into C++.
Then a native C++ compiler compiles that generated C++ code for the target platform.
The simplified process becomes:
C# → IL → IL2CPP → C++ → native compiler → native machine code
By the time your application reaches the user's device, the required native code has already been compiled for the target platform.
There is no need for Mono to JIT-compile that managed code when the application starts.
That is the core idea behind IL2CPP.
What Actually Happens During an IL2CPP Build?
This is where the name IL2CPP starts making sense.
IL2CPP means:
Intermediate Language To C++
When you build with IL2CPP, Unity roughly goes through these stages:
- Your C# code is compiled into managed assemblies containing IL.
- Unity performs managed code stripping.
- IL2CPP converts the managed assemblies into C++.
- The generated C++ code and IL2CPP runtime are compiled using the platform's native compiler.
- The resulting native code is linked into the final application.
So IL2CPP isn't a magical C# compiler.
It is a conversion and compilation pipeline.
Unity takes your managed code, turns it into C++, then lets the native platform toolchain produce the final native binary.
That extra work is one reason IL2CPP builds can take significantly longer than Mono builds.
Why Can't We Just Use Mono Everywhere?
Because platforms have rules.
Some platforms don't allow applications to generate executable code at runtime in the way JIT requires.
Apple's iOS environment is the classic example.
Unity therefore needs an AOT approach for platforms where JIT compilation isn't allowed.
IL2CPP solves this by making the native code during the build instead of trying to generate it later on the device.
This is one of the biggest reasons IL2CPP exists.
It wasn't created simply because Unity developers wanted another checkbox in Player Settings. Humanity already has enough checkboxes.
It exists because different platforms impose different technical restrictions.
Mono vs IL2CPP
Now the difference becomes much easier to understand.
| Mono | IL2CPP | |
|---|---|---|
| Compilation model | JIT | AOT |
| Native code generated | During runtime | During build |
| C++ conversion | No | Yes |
| Build time | Usually faster | Usually slower |
| Iteration speed | Usually better | Usually worse |
| Runtime JIT required | Yes | No |
| Platform compatibility | More limited | Much wider |
| Dynamic code | More flexible | More restricted |
| Native binary | Not fully precompiled in the same way | Contains native compiled code |
| Typical use | Development and supported desktop platforms | Mobile, consoles and many production builds |
The table is useful, but don't turn it into a religion.
The correct backend depends on the target platform and the requirements of your project.
Security and Reverse Engineering IL2CPP: One another big reason to pick IL2CPP is that: it's way harder to steal your code.
With Mono, someone can open your .dll in a free tool like dnSpy/dotpeek and get back your original C#, readable, with variable names and logic intact. IL2CPP compiles to native machine code instead, so there's no clean source to recover. Reverse engineers are stuck reading raw assembly instead of your actual code.
One caveat: IL2CPP builds still include a metadata file(global-metadata.dat) with your class and method names, so tools like Il2CppDumper can figure out your API's shape. But the actual logic inside each method stays hidden. If you want the names hidden too, you need to add metadata obfuscation on top.
Bottom line: IL2CPP makes your code way harder to rip off, even without extra tools — something pure performance comparisons usually miss.
Why Is Mono Usually Faster to Build?
This is one of the biggest practical differences you will notice as a Unity developer.
With Mono, Unity can compile your C# into managed assemblies and rely on the Mono runtime to JIT-compile the required code when the application runs.
IL2CPP has considerably more work to do.
Unity has to:
C# → IL → C++ → C++ compilation → linking → final binary
That generated C++ can be large.
Then the native compiler has to compile it.
Then the linker has to put everything together.
Large Unity projects can therefore have painful IL2CPP build times.
This becomes especially noticeable when your project contains a lot of scripts, packages and generic code.
For day-to-day development, this matters.
If you are repeatedly making small C# changes and rebuilding an IL2CPP player every few minutes, you are voluntarily turning your development cycle into a waiting simulator.
Use the fastest workflow that makes sense.
Does IL2CPP Make Your Game Faster?
This is where developers often oversimplify things.
You will hear:
"IL2CPP is faster than Mono."
That's not a good rule.
IL2CPP can provide performance benefits because the application doesn't need to JIT managed code at runtime. Unity also generates native code that the platform's native compiler can optimize.
Startup can be better because there is no JIT compilation phase for the managed code.
But don't expect IL2CPP to magically turn a badly written game into a fast one.
If your game spends most of its frame time doing this:
for (int i = 0; i < 1000000; i++)
{
Instantiate(enemyPrefab);
}
changing the scripting backend isn't going to rescue your architecture.
You still have bad code.
Profile first.
Optimize the actual bottleneck.
Then compare Mono and IL2CPP when the difference matters.
IL2CPP Is Not Burst
This distinction is worth making because these technologies are often mixed together.
IL2CPP is a scripting backend.
It determines how Unity's managed code gets turned into executable code for the target platform.
Burst is a compiler designed for specific performance-oriented code, especially code using Unity's Job System and Burst-compatible APIs.
They solve different problems.
You can use IL2CPP and Burst together.
For example:
C# → IL2CPP → native code
while selected Burst-compatible jobs can be compiled by Burst using its own compilation pipeline.
Don't think of IL2CPP as "Unity's performance compiler."
It isn't.
The Biggest AOT Problem: Code That Only Exists at Runtime
AOT compilation creates an important restriction.
The compiler needs to know what code needs to exist before the application runs.
This becomes interesting when your application starts doing things dynamically.
For example:
- Reflection
- Dynamically generated methods
- Runtime code generation
- Some generic types and methods
- Native-to-managed callbacks
- Dynamically loaded assemblies
- Code that is only referenced indirectly
Mono can often handle some of these cases more naturally because it can compile required code at runtime.
IL2CPP cannot simply wait for a missing native implementation to appear later.
The code has already been compiled.
This is why you can sometimes get an application that works perfectly with Mono but fails after switching to IL2CPP.
The problem isn't necessarily your C# code.
The problem may be that your code depends on something the AOT build could not determine or generate correctly.
Reflection and Code Stripping
This is especially important in real Unity projects.
Suppose you have:
var type = Type.GetType("MyGame.PlayerData");
You might never directly reference PlayerData anywhere else.
A code stripping system may decide:
"Nothing appears to use this type."
And remove it from the final build.
Now your code asks for it at runtime.
It isn't there.
Congratulations. Your code worked perfectly in the Editor.
Production has other opinions.
This is why reflection-heavy systems, serializers, dependency injection frameworks and dynamically accessed types need proper testing with the actual target backend.
Unity provides mechanisms such as [Preserve] and link.xml to prevent required code from being stripped.
The exact solution depends on the library and how the code is accessed.
Don't blindly add [Preserve] to everything.
That defeats stripping and can increase your build size.
Fix the actual dependency.
Generics Can Also Matter
Generics are another area where AOT can expose problems.
Consider:
public T Create<T>()
{
// ...
}
With JIT, the runtime has the ability to generate native code for generic instantiations when they are needed.
With AOT, Unity has to generate the required native implementations during the build.
IL2CPP uses techniques such as generic sharing to reduce the amount of generated code, but there are still cases where the compiler cannot determine every runtime usage.
This can lead to problems involving generic methods or types that are only reached indirectly.
Again, the practical lesson is simple:
Test IL2CPP builds.
Don't assume that because something works in the Editor, it will behave identically in an AOT player.
What About Debugging?
Mono has traditionally been the more comfortable backend for rapid development.
IL2CPP has a reputation for being difficult to debug because the generated C++ code looks terrifying.
And honestly, it can.
But you generally don't need to debug the generated C++ code directly.
Unity supports managed-code debugging with IL2CPP.
So your normal C# debugging workflow does not simply disappear because you selected IL2CPP.
The real difference is that when something goes wrong specifically because of AOT, stripping, native interop or platform restrictions, the problem can become more complicated to diagnose.
This is another reason to test the actual production backend early.
Don't wait until the night before release to discover that your serializer has been having an existential crisis with IL2CPP.
So Which One Should You Use?
For me, the answer is simple.
Use Mono when you are developing and your target platform supports it.
Use IL2CPP when your target platform requires it or when you specifically want the characteristics of an AOT native build. And most important when you need security and want to create hurdles in reverse engineering as it increases the code security.
For many Unity projects, a practical workflow is:
Development → Mono
Production/mobile/console → IL2CPP
But don't treat this as a universal rule.
If your project targets iOS, consoles, or another platform where IL2CPP is required, use IL2CPP during development too when you need to test platform-specific behavior.
And if your project relies heavily on reflection, dynamic loading, runtime code generation or unusual native interop, test IL2CPP early.
The later you discover an AOT problem, the more expensive it becomes.
The Real Difference You Should Remember
Don't memorize:
Mono = good
IL2CPP = good
That tells you almost nothing.
Remember this instead:
Mono uses JIT.
The application can compile managed code into native code while it runs.
IL2CPP uses AOT.
Unity converts managed IL into C++, compiles it ahead of time, and ships native code for the target platform.
From that one difference, most of the other behavior follows.
JIT gives you more runtime flexibility and usually faster iteration.
AOT gives you broader platform compatibility and removes the need for runtime JIT compilation.
AOT also means more work during the build, more restrictions around dynamically generated code, and more reasons to test your real player build instead of trusting the Unity Editor.
That is the part developers actually need to understand.
The dropdown in Player Settings is just the final decision.
The real decision is:
Do I want code compiled when I build the application, or do I want part of that work to happen when the application runs?
Once you understand that, Mono vs IL2CPP stops being a mysterious Unity setting and becomes a straightforward engineering trade-off.
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