The conventional procedure for preparing biological samples to be processed by the conventional method of paraffin embedding, staining with Hematoxylin and Eosin, permanent mounting, interpretation of results, evaluation of possible artifacts and biological activity is presented. The procedure is adapted according to the nature of the sample to be processed: invertebrate, vertebrate, plant, fungus, marine organism. I am not authorized to process biopsies, which corresponds to a pathologist or his technician with credentials from the health authorities. Bivalve gonadal tissue is used as a model for the identification of cell types, according to sex and stages of sexual maturity.