Returns the angle in radians whose Tan is y/x.

Return value is the angle between the x-axis and a 2D vector starting at zero and terminating at (x,y).

Note that this function takes account of the cases where x is zero and returns the correct angle rather than throwing a division by zero exception.

	// Usually you use transform.LookAt for this.
	// But this can give you more control over the angle
	var target : Transform;
	function Update () {
		var relative : Vector3  = transform.InverseTransformPoint(target.position);
		var angle : float = Mathf.Atan2(relative.x, relative.z) * Mathf.Rad2Deg;
		transform.Rotate (0, angle, 0);
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    public Transform target;
    void Update() {
        Vector3 relative = transform.InverseTransformPoint(target.position);
        float angle = Mathf.Atan2(relative.x, relative.z) * Mathf.Rad2Deg;
        transform.Rotate(0, angle, 0);
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	public target as Transform

	def Update() as void:
		relative as Vector3 = transform.InverseTransformPoint(target.position)
		angle as float = (Mathf.Atan2(relative.x, relative.z) * Mathf.Rad2Deg)
		transform.Rotate(0, angle, 0)