Quaternion.LookRotation

Parameters

Description

Creates a rotation with the specified forward and upwards directions.

Z axis will be aligned with forward, X axis aligned with cross product between forward and upwards, and Y axis aligned with cross product between Z and X.
If used to orient a Transform, the Z axis will be aligned with forward/ and the Y axis with upwards if these vectors are orthogonal.

// You can also use transform.LookAt

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    public Transform target;

    void Update()
    {
        Vector3 relativePos = target.position - transform.position;

        // the second argument, upwards, defaults to Vector3.up
        Quaternion rotation = Quaternion.LookRotation(relativePos, Vector3.up);
        transform.rotation = rotation;
    }
}

See Also: SetLookRotation.