The number following the USP is the edition. The current edition is USP26/NF21. On January 1 2004 the newest edition USP27/NF22 becomes current. USP stands for United States Pharmacopeia , NF stands for National Formulary. Most pharmaceutical actives (the active ingredients) have a monograph (monograph = list of tests and requirements the substance must have to be sold in medicines under FDA jurisdiction), most pharmaceutical excipients (the other stuff in pharmaceuticals such as preservatives, fillers, buffers, thickeners etc.) have a monograph contained in the NF. Back in the olden days a new Pharmacopeia only came out once every few years, these days things are changing very fast and in an effort to keep things up to date a new edition comes out every year, and several updates come out in between, and between those other smaller updates are included in the PF (Pharm Forum). Basically things change almost daily, therefore most pharmaceutical companies have entire departments whose sole purpose is to make sure thier lab complies with all the constantly changing requirements. As far as what that means to you, if you see a pharmaceutical or chemical that says USP23 versus one that says USP26, it simply means that the USP23 chemical was tested using an older set of specifications. It does not mean that the chemical is inferior in quality. Since most Monographs don't change from edition to edition, if it was Ok to be called USP 25 its would most likely pass all the tests for USP 26 or vice versa. It may however mean a chemical has been sitting on a shelf for a while. Also it is important to note that many countries other than the USA sometimes use the USP, and therefore might use an older edition. Another factor that sometimes comes into play is the cost of testing the chemical, it might be more expensive than the value of the chemical itself to perform the tests again to obtain the current rating. Hope this helped. After writing this I have decided to write a post about Pharmacopeias For Dummies to put all questions to bed once and for all.