Many Penicillium species produce mycotoxins. The importance of these toxic compounds varies widely, and is governed as much by the biology and ecology of the species concerned as by the inherent toxicity of the compounds themselves. For example, P. citreonigrum and P. islandicum make potent toxins, but as both species are rare in nature, the toxins are not important. Although P. janthinellum and P. simplicissimum are very widely distributed and make potent toxins, these species are rarely found outside soils so again, the toxins are of little practical importance. The very common P. crustosum makes a potent tremorgenic mycotoxin, fortunately, the toxin is only produced at very high water activities. On the other hand, P. verrucosum, unknown in the tropics, is widespread in cereals in cold climates. Consequently, ochratoxin A production by this species causes a major toxicosis. The biology and ecology of these and other Penicillium mycotoxins will be described in this paper.