No, not all K cups fit all Keurigs. There are three different types of K cup brewers and each one uses a different type of K cup. The Original line of Keurig brewers use the small size K cup, the Vue and the Plus series use the large size K cup, and the 2.0 brewer uses a unique K cup that has both a small and large opening. If you try to use a K cup that is not compatible with your brewer, it will not work properly and could damage your machine.