molecular level. Unique molecules produced by unusual pathways of these parasites have also been discovered and found to play important roles in their survival in the host. Understanding these mechanisms and pathways is essential not only to formulate a rational strategy for chemo- and immuno-prophylaxis and -therapy but also to unravel the mystery of biological evolution in symbiosis and parasitism. In the advent of our knowledge on the molecular biology and biochemistry of parasite membrane and other molecules, it is opportune to examine and discuss their possible roles in host-parasite recognition and interaction in a comparative approach. To highlight the recent advances of this area in various host- parasite systems, a NATO advanced Research Workshop was held from September 27 to October 1, 1986 at Hotel Villa del Mare, Acquafredda di Maratea, Italy.