Water filtration is a bit confusing. Stages just refers to the number of different kinds of filtration the water goes through before being output. Typically there are only 4 types of filtration: pre or particle, carbon, reverse osmosis (RO), and di-ionization (DI). I wouldn't use DI for drinking water (usually used only in labs or for things like salt water fish tanks) so there is only really 3 possible types of filters used. That said, some manufacturers use multiple stages of the same type. So they might go from pre filter to dual carbon to RO to carbon. This would make a 5 stage even though it's only using 3 types of filters. Confusing enough? Personally I only use a simple Brita sink mounted filter now although I still own an RO setup from when I had a fish tank. There are those that argue that you shouldn't remove minerals from the water since they are necessary. This is referred to as selective filtration - removing only things like lead and other harmful things. Selective filtration can be done easily and inexpensively with a sink mounted filter like Pur or Brita. If you insist/need RO, you might want to give Charles Gibbs a call from SpectraPure 1.800.685.2783 http://www.spectrapure.com . He is really knowledgeable and can talk you though filtration for your needs.