The microorganisms that can be injurious to beer are reviewed and discussed. The identification of the various microorganisms by simple techniques and the importance of their proper identification are highlighted. Microscopic examination of pitching yeast, culturing in semisolid media, the membrane-filter technique, culturing in liquid media, and forcing tests are critically reviewed. The value of the Jorgensen method for detection of wild yeasts and of the forcing tests, or extended incubation tests, is emphasized. A detailed sampling program incorporating the various methods discussed is outlined in tabular form.