For the first time, the monograph summarizes information on the causes of errors that occur in the works of histologists and pathologists. Based on our own experience and instructions scattered in the literature, the authors consistently consider artifacts arising from the receipt and processing of material for microscopic examinations, agonal and early postmortem changes, some features of the normal structure of organs and tissues that misled insufficiently experienced researchers, as well as the most frequent morphological manifestations of transferred or latent diseases in humans and laboratory animals.