Mixins are a powerful and important tool used in the Fabric ecosystem and other modding frameworks for Minecraft. Their primary use case is modifying existing code in the base game, whether it be through injecting custom logic, removing mechanics, or modifying values. Note that only Mixins written in Java are directly supported, even if you use Kotlin or another language running on the Java Virtual Machine for the rest of your mod. Whilst it may be technically possible to use other JVM languages apart from Kotlin which is explicitly unsupported, there are currently no plans to support it or document such a process.

Mixin is a complex subsystem which merges a mod's Mixin “classes” ¹⁾ into the targeted classes' compiled Bytecode at runtime by parsing the mixin classes' compiled bytecode. It will merge both the annotated methods, and other members and interfaces added to the Mixin class. The specifics of how this is done is outside the scope of this introduction, refer to the relevant Mixin Wiki links on this page for details.

It is not recommended to try and learn how Mixin works or how to use it without having some notion of what Bytecode is, as that may lead to unexpected behavior and misunderstandings. It is also recommended to know how to read Java Bytecode for your IDE of choice, as reading portions of the Bytecode may sometimes be necessary to use Mixin properly, notably when targeting anonymous classes, lambdas or code with a lot of if-else logic branching.
Similarly, Mixin and this documentation expects the user to already be familiar with reading Minecraft and external source code, and be able to set up an initial development environment, aswell as having a decent understanding of Java fundamentals. If, however, you specifically wish to skip to examples, be aware that it is expected for you to encounter difficulties and confusion in your learning process as you try to tackle more technical issues without learning some technical aspects, and you are likelier to need direct support.
It is recommended to leverage the Minecraft Development plugin by demonwav on IntelliJ (AKA MCDev) for Mixin development to benefit from its autocompletion and utilities such as the MixinExtras Flow Diagram.

Mixin can do its work throughout the game's lifecycle, however, a mixin must be applied before the targeted class has loaded. For the majority of mixins, this is very early in the overall Fabric launching process, but do mind that the game launching without Mixin crashing or logging warns doesn't mean all changes have applied as intended.

There are different tools Mixin and MixinExtras ²⁾ provide for modifying source code. Those tools are usable by the users in the form of Java annotations, those annotations decorate(annotate, are attached to) members of the Mixin class, itself decorated and registered, and are passed metadata which is used by Mixin to process the Mixin class. The class's members, with some exceptions, are then merged into the target class. Here is a basic overview of some of Mixin's important tools:
There ought to be dedicated pages going more in depth about the different tools annotations later on. This segment as it is edited should be kept to a cursory overview of Mixin(Extras) features.

• The @Mixin annotation decorates every Mixin class. It mainly dictates the class to target and the priority of the decorated class relative to other mixin classes with the same target.
• Injector Mixin annotations decorate a handler method³⁾ and instruct Mixin to merge it into the target class and then call it at the injection point defined within the annotation, with additional instructions around the call depending on the injector. Injectors are distinguished by the fact they do not overwrite the targeted bytecode⁴⁾, injectors include all method annotations from MixinExtras, and in base Mixin mainly @Inject, @ModifyVariable or @ModifyArg.⁵⁾ Injectors should be treated your primary way to modify external code as they are more compatible (more than one mod can inject the same target without outright failing), and MixinExtras provides injector alternatives that fulfill nearly all roles of non-injector tools such as redirectors/redirect injectors and overwriting.
• Redirector Mixin annotations refers to annotations that merge the handler method and replace the targeted injection point with a callback to your handler method. Think of it as overwriting what you targeted and replacing it with a call to the decorated handler method. Whilst a name for them is also Redirect Injectors⁶⁾, you should generally treat them differently as they do not merely inject but also overwrite information and cannot chain. The main redirectors are @Redirect and @ModifyConstant. These should only be used to cause an incompatibility from another mod trying to do the same on the same target.
• Overwriting, by using the @Overwrite annotation simply replaces the targeted method with your own. There is practically never any reason to use this as a mod developer, and you should always prioritize other solutions like MixinExtras' @WrapMethod or more specific modifications in the targeted method. If you think you are in a situation that requires overwriting, it is advised to first consult support channels for a second opinion.
• One of Mixin's most fundamental features, upon which most of its tools depend, is merging a Mixin class's members into the target class. This includes interface implementations and most members (with some exceptions). Annotations such as @Inject serve as a way to describe special behavior to be done on top or instead of merging the decorated member.

For more complete and thorough information on Mixin functionality, usage, and mechanics, view the Mixin Official Wiki. Documentation specific to individual Mixin features can be found in the Mixin Javadoc. MixinExtras features are well-documented on its Official Wiki. JavaDoc documentation of both may be found in your local development environment with Mixin set up aswell.

The Fabric Wiki offers several articles that provide practical examples and explanations of some areas of the Mixin and MixinExtras tools:

• Mixin registration
• Injects
• Accessors and Invokers
• Redirectors
  • Method redirectors
• Tips (WIP)
• Examples
• Hotswapping Mixins
• Exporting Mixin Classes
• Access Wideners
• Reflection
• Interface Injection
• Modding Tips