Another great question!99.99% of the heads go with 99.99% of the cartridges...there are a few quirky exceptions but the lion's share are interchangeable; the that's the beauty of Everpure.  Once you have the head, the world is your oyster...you can try all sorts of cartridges.  As long as you have the room to accommodate the length of the cartridge, you're golden.The quirky exceptions revolve around a line of products that fit into what is called a 'BW" head.  On our site, the only products we carry that fit into a BW head are the Pro Series (1500, 2500, 3500) and the Quantum 1000 & 4000.Click here for a reference chart which provides options for which cartridge can be used based on what you are using now.  Please note this chart is for residential drinking water systems.