Parasitism is a way of life, more distributed in the living world than it is generally considered. During the evolution host-parasite associations developed in which the mode of nutrition of the parasite causes production losses, sometime diseases to the host organisms. Practically all invertebrate and vertebrate species have their own set of parasites as „uninvited companion”. They include unicellular species, worms (flukes, tapeworms, roundworms) as well as arthropods. Through examples selected from the highly interesting world of parasites an insight will be given to the audience into the mayor types of biology, damaging effects of the parasites, and into the methods of their control.