The need for adequate preservation and the difficulties of definition and testing are outlined and pharmacopoeial challenge tests are compared. While similar in principle they differ in detail and in the interpretation of results. The USP test is intended for multi-dose sterile products but BP and draft EP tests include also criteria for topical and liquid oral preparations. Those of the BP, based on the performance of established well-preserved BP preparations, are not mandatory but provide guidance for the selection of suitable antimicrobial preservatives for inclusion in formulae of the Pharmacopoeia.