This new se ries is concerned with intercellular communication and recognition. It is now widely appreciated that these processes playa crucial role in virtually all biological systems and functions. These encompass fertilisation, embryonic development, infectious interactions, the activity of the nervous system, the regulation of growth and development by hormones and the immune response to foreign or 'non-self antigens. Historically as described in the first review in this volume, the general concept of cell-associated receptors as the molecular entity primarily responsible for the specificity of signal recognition arose independently in the fields of immunology, pharmacology and developmental biology. From an early stage the analogy between cellular recognition and the discriminatory activity of antibodies and enzymes was emphasised. A vital conceptual advance, expressed most c1early by Linus Pauling and Paul Weiss, was the idea that non-covalent molecular interactions (of proteins in particular) were responsible forbiological specificity in in general. In the last decade several major advances have led to a new level of understanding of the molecular basis of cellular recognition. In several systems (in particular with neurotransmitters, hormones and antigens) it is possible to direct1y demonstrate the existence of receptors - associated in each case with the cell surface. These studies have been paralleled by equally important insights into the general structure and organisation of cell membranes and the possible ways in which signals arriving from the 'outside' can be transduced across the cell surface membrane to induce or regulate the cell's programmed responses.