MonoBehaviour

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The MonoBehaviour class is the base class from which every Unity script derives, by default. When you create a C# script from Unity’s project windowA window that shows the contents of your Assets folder (Project tab) More info
See in Glossary, it automatically inherits from MonoBehaviour, and provides you with a template script. See Creating and Using scripts for more information on this.

The MonoBehaviour class provides the framework which allows you to attach your script to a GameObject in the editor, as well as providing hooks into useful Events such as Start and Update.

For a complete reference of every member of the MonoBehaviour class, and its technical details, see the MonoBehaviour script reference.

The script inspector

When you select a script in the Project window, the InspectorA Unity window that displays information about the currently selected GameObject, asset or project settings, allowing you to inspect and edit the values. More info
See in Glossary displays some basic information about that script asset, including the name of the assembly it belongs to, and a preview of the contents of the script.

Note: Although the Inspector displays the contents of the script, you can’t edit the script in the Inspector window.

The script inspector also displays two buttons, Open and Execution Order.

The Open button opens the script in the currently configured External Script Editor (also known as an Integrated Development Environment, or IDE). The Open button performs the same function as double-clicking the script in the Project window. You can configure which external editor Unity uses to open your scriptsA piece of code that allows you to create your own Components, trigger game events, modify Component properties over time and respond to user input in any way you like. More info
See in Glossary in the External Tools section of the Preferences window.

The Execution Order button opens the Script Execution Order section of the Project Settings window, which allows you to alter the the order in which Unity executes your scripts.

Default object references

If you define public ObjectThe fundamental object in Unity scenes, which can represent characters, props, scenery, cameras, waypoints, and more. A GameObject’s functionality is defined by the Components attached to it. More info
See in Glossary fields that can be assigned in the Editor in your MonoBehaviour script, you can set up default references for these fields. The default reference fields are visible in the inspector when you select the script asset in the Project window.

In the example above, there are three public Audio ClipA container for audio data in Unity. Unity supports mono, stereo and multichannel audio assets (up to eight channels). Unity can import .aif, .wav, .mp3, and .ogg audio file format, and .xm, .mod, .it, and .s3m tracker module formats. More info
See in Glossary fields, without default references assigned. You could assign audio clips to each of the AudioClip default reference fields.

If you assign default references, they are applied when you add your MonoBehaviour as a component to a GameObject, or when you reset an existing instance of your MonoBehaviour on a GameObject to its default values.

Note: There is no ongoing link between the references on MonoBehaviour instances on GameObjects and the default references. This means if you change the default references, they are not automatically updated on existing GameObjects.

Other types of inspector-editable fields that do not inherit from UnityEngine.Object (for example, public string or int fields) do not have default fields in the inspector. Instead, they take their default values from the script itself.

Coroutines

The MonoBehaviour class allows you to start, stop, and manage Coroutines, which are a way to write asynchronous code which can include waiting for a certain amount of time, or for certain actions to complete, while allowing other code to continue executing.

For more information about coroutines, see the Coroutines manual page and the StartCoroutine method script reference.

Events

The MonoBehaviour class provides access to a large collection of event messages, which allows you to execute your code based on what is currently happening in your project. Here are a few of the more common examples. For a list of them all, see the Messages section on the MonoBehaviour script reference page

Start - called when the GameObject begins to exist (either when the Scene is loaded, or the GameObject is instantiated).

Update - called every frame.

FixedUpdate - called every physics timestep.

OnBecameVisible and OnBecameInvisible - called when a GameObject’s renderer enters or leaves a cameraA component which creates an image of a particular viewpoint in your scene. The output is either drawn to the screen or captured as a texture. More info
See in Glossary’s view.

OnCollisionEnter and OnTriggerEnter - called when physics collisionsA collision occurs when the physics engine detects that the colliders of two GameObjects make contact or overlap, when at least one has a Rigidbody component and is in motion. More info
See in Glossary or triggers occur.

OnDestroy - called when the GameObject is destroyed.