Collisions and triggers

Quick overview

A collision callback is for physical contact between colliders; a trigger callback is for overlap detection without normal collision response. The callbacks have distinct parameter types and prerequisites. This page covers Unity’s 3D physics; the 2D methods have a 2D suffix.

Message	Use it for
`OnCollisionEnter(Collision collision)`	First frame of physical contact
`OnCollisionStay(Collision collision)`	Continued contact
`OnCollisionExit(Collision collision)`	Contact ended
`OnTriggerEnter(Collider other)`	Another collider entered a trigger
`OnTriggerStay(Collider other)`	Another collider remains in a trigger
`OnTriggerExit(Collider other)`	Another collider left a trigger

Trigger example

using UnityEngine;

public sealed class PickupZone : MonoBehaviour
{
    private void OnTriggerEnter(Collider other)
    {
        if (!other.CompareTag("Player")) return;
        Debug.Log("The player entered the pickup zone.");
    }
}

Add a collider to the zone, enable Is Trigger, and ensure the two objects meet Unity’s Rigidbody and collider requirements for a trigger event. Do not assume that any pair of colliders will generate a message.

Physical collision example

using UnityEngine;

public sealed class ImpactReport : MonoBehaviour
{
    private void OnCollisionEnter(Collision collision)
    {
        Debug.Log($"Hit {collision.gameObject.name}");
    }
}

Troubleshooting checklist

Check whether the colliders are enabled and set to Is Trigger for the intended behavior.
Confirm the relevant object has a compatible Rigidbody and that the 3D APIs are being used with 3D components.
Check the Layer Collision Matrix in Physics settings.
Make sure the callback signature is exact, including parameter type and capitalization.
Test a minimal two-object scene before adding complex gameplay rules.

Read Rigidbody and movement for the movement side of the interaction.