The Gram positive bacteria are generally regarded as the most hazardous beer spoilage organisms in modern breweries, especially the lactobacilli: L. brevis, L. lindneri, L. curvatus, L. casei, L. buchneri, L. coryneformis, L. plantarum, L. brevisimilis, L. malefermentans and L. parabuchneri and the pediococci: P damnosus, P. inopinatus and P. dextrinicus. Micrococcus kristinae is the only species within the micrococci relevant to brewing. The Gram negative strictly anaerobic bacteria are apparently increasing in importance and include Pectinatus cerevisiiphilus, Pectinatus frisingensis and Selenomonas lacticifex, reported as obligate beer spoilage organisms: Zymophilus raffinosivorans as a potential beer spoilage organism; Megasphaera cerevisiae as an obligate spoilage organism of low alcohol beer and Zymomonas mobilis as capable of spoiling primed beer. With improved process technology the importance of aerobic bacteria has decreased and the same applies for the Gram negative aerobic bacteria Hafnia protea and Enterobacter cloacae which are capable of surviving beer fermentation. Beer spoilage organisms include several so-called wild yeasts, of which Saccharomyces species are generally considered the most important. Even though the detection of beer spoilage organisms by cultivation in laboratory media does not always provide the specificity and the sensitivity required, the use of selective media and incubation conditions still appears to be the method preferred by breweries. The media used depend on the type of sample, the specificity required and, for detection of wild yeasts, to some extent, the characteristics of the culture yeast. Among the media reported so far no single medium can be used to detect all members within a group of specific beer spoilage organisms and further work on the development of improved substrates are required both for bacteria and wild yeasts.