Physics.Raycast
static function Raycast(origin: Vector3, direction: Vector3, distance: float = Mathf.Infinity, layerMask: int = DefaultRaycastLayers): bool;
static bool Raycast(Vector3 origin, Vector3 direction, float distance = Mathf.Infinity, int layerMask = DefaultRaycastLayers);
static def Raycast(origin as Vector3, direction as Vector3, distance as float = Mathf.Infinity, layerMask as int = DefaultRaycastLayers) as bool
Parameters

origin The starting point of the ray in world coordinates.
direction The direction of the ray.
distance The length of the ray.
layerMask A Layer mask that is used to selectively ignore colliders when casting a ray.
Returns
bool True when the ray intersects any collider, otherwise false.
Description

Casts a ray against all colliders in the scene.

	function Update () {
		var fwd = transform.TransformDirection (Vector3.forward);
		if (Physics.Raycast (transform.position, fwd, 10)) {
			print ("There is something in front of the object!");
		}
	}
using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Update() {
        Vector3 fwd = transform.TransformDirection(Vector3.forward);
        if (Physics.Raycast(transform.position, fwd, 10))
            print("There is something in front of the object!");
        
    }
}
import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Update() as void:
		fwd as Vector3 = transform.TransformDirection(Vector3.forward)
		if Physics.Raycast(transform.position, fwd, 10):
			print('There is something in front of the object!')

Notes: This function will return false if you cast a ray from inside a sphere to the outside; this in an intended behaviour. If you move colliders from scripting or by animation, there needs to be at least one FixedUpdate executed so that the physics library can update it's data structures, before a Raycast will hit the collider at it's new position.
static function Raycast(origin: Vector3, direction: Vector3, hitInfo: RaycastHit, distance: float = Mathf.Infinity, layerMask: int = DefaultRaycastLayers): bool;
static bool Raycast(Vector3 origin, Vector3 direction, RaycastHit hitInfo, float distance = Mathf.Infinity, int layerMask = DefaultRaycastLayers);
static def Raycast(origin as Vector3, direction as Vector3, hitInfo as RaycastHit, distance as float = Mathf.Infinity, layerMask as int = DefaultRaycastLayers) as bool
Parameters

origin The starting point of the ray in world coordinates.
direction The direction of the ray.
distance The length of the ray.
hitInfo If true is returned, hitInfo will contain more information about where the collider was hit (See Also: RaycastHit).
layerMask A Layer mask that is used to selectively ignore colliders when casting a ray.
Returns
bool True when the ray intersects any collider, otherwise false.
Description

Casts a ray against all colliders in the scene and returns detailed information on what was hit.

	function Update () {
		var hit : RaycastHit;
		if (Physics.Raycast (transform.position, -Vector3.up, hit)) {
			var distanceToGround = hit.distance;
		}
	}
using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Update() {
        RaycastHit hit;
        if (Physics.Raycast(transform.position, -Vector3.up, out hit))
            float distanceToGround = hit.distance;
        
    }
}
import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Update() as void:
		hit as RaycastHit
		if Physics.Raycast(transform.position, -Vector3.up, ):
			distanceToGround as float = hit.distance

Another example:
	// Raycast up to 100 meters down

function Update () { var hit : RaycastHit; if (Physics.Raycast (transform.position, -Vector3.up, hit, 100.0)) { var distanceToGround = hit.distance; } }
using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Update() {
        RaycastHit hit;
        if (Physics.Raycast(transform.position, -Vector3.up, out hit, 100.0F))
            float distanceToGround = hit.distance;
        
    }
}
import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Update() as void:
		hit as RaycastHit
		if Physics.Raycast(transform.position, -Vector3.up, , 100.0F):
			distanceToGround as float = hit.distance

static function Raycast(ray: Ray, distance: float = Mathf.Infinity, layerMask: int = DefaultRaycastLayers): bool;
static bool Raycast(Ray ray, float distance = Mathf.Infinity, int layerMask = DefaultRaycastLayers);
static def Raycast(ray as Ray, distance as float = Mathf.Infinity, layerMask as int = DefaultRaycastLayers) as bool
Parameters

ray The starting point and direction of the ray.
distance The length of the ray.
layerMask A Layer mask that is used to selectively ignore colliders when casting a ray.
Returns
bool True when the ray intersects any collider, otherwise false.
Description

Same as above using /ray.origin/ and /ray.direction/ instead of origin and direction.

function Update() {
	var ray = Camera.main.ScreenPointToRay (Input.mousePosition);

if (Physics.Raycast (ray, 100)) { print ("Hit something"); } }
using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Update() {
        Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
        if (Physics.Raycast(ray, 100))
            print("Hit something");
        
    }
}
import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Update() as void:
		ray as Ray = Camera.main.ScreenPointToRay(Input.mousePosition)
		if Physics.Raycast(ray, 100):
			print('Hit something')

static function Raycast(ray: Ray, hitInfo: RaycastHit, distance: float = Mathf.Infinity, layerMask: int = DefaultRaycastLayers): bool;
static bool Raycast(Ray ray, RaycastHit hitInfo, float distance = Mathf.Infinity, int layerMask = DefaultRaycastLayers);
static def Raycast(ray as Ray, hitInfo as RaycastHit, distance as float = Mathf.Infinity, layerMask as int = DefaultRaycastLayers) as bool
Parameters

ray The starting point and direction of the ray.
distance The length of the ray.
hitInfo If true is returned, hitInfo will contain more information about where the collider was hit (See Also: RaycastHit).
layerMask A Layer mask that is used to selectively ignore colliders when casting a ray.
Returns
bool True when the ray intersects any collider, otherwise false.
Description

Same as above using /ray.origin/ and /ray.direction/ instead of origin and direction.

function Update() {
	var ray = Camera.main.ScreenPointToRay (Input.mousePosition);
	var hit : RaycastHit;
	if (Physics.Raycast (ray, hit, 100)) {
		Debug.DrawLine (ray.origin, hit.point);
	}
}
using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Update() {
        Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
        RaycastHit hit;
        if (Physics.Raycast(ray, out hit, 100))
            Debug.DrawLine(ray.origin, hit.point);
        
    }
}
import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Update() as void:
		ray as Ray = Camera.main.ScreenPointToRay(Input.mousePosition)
		hit as RaycastHit
		if Physics.Raycast(ray, , 100):
			Debug.DrawLine(ray.origin, hit.point)