Physics.Raycast
static function Raycast(origin: Vector3, direction: Vector3, distance: float = Mathf.Infinity, layerMask: int = DefaultRaycastLayers): 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!");
		}
	}
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;
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;
		}
	}
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; } }
static function Raycast(ray: Ray, distance: float = Mathf.Infinity, layerMask: int = DefaultRaycastLayers): 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"); } }
static function Raycast(ray: Ray, hitInfo: RaycastHit, distance: float = Mathf.Infinity, layerMask: int = DefaultRaycastLayers): 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);
	}
}