IComparable interface not found C#

I need a comparable class, so I created something based on the info in this question: Sorting builtin arrays

My class looks like this:

using UnityEngine;
using System.Collections.Generic;

public class WayPoint : IComparable<WayPoint> {
//...
    public int CompareTo(WayPoint other) {
        return index.CompareTo(other.index);
    }
}

But for some reason the IComparable interface is not found:

The type or namespace name `IComparable`1' could not be found. Are you missing a using directive or an assembly reference?

I'm using Unity 3.3. Did the IComparable interface move somewhere else, or does it not exist anymore?

IComparable lives in the System namespace, so you need:

using System;