Quaternion.FromToRotation

Description

Creates a rotation which rotates from fromDirection to toDirection.

Usually you use this to rotate a transform so that one of its axes eg. the y-axis - follows a target direction toDirection in world space.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    void Example() {
        transform.rotation = Quaternion.FromToRotation(Vector3.up, transform.forward);
    }
}