The thymus has been recognized as a site of primary immune function for more than three decades and yet, despite intense research, many of the central questions regarding its function in T-lymphocyte maturation have remained unanswered. Current cellular and molecular analytical techniques are finally proving to be sophisticated enough to define the cells which comprise the thymic framework and dissect their complex interactions with developing thymocytes. In a series of articles over the coming months Immunology Today will be investigating the complex interrelationship between the thymic microenvironment and the developing T cell. As a foundation for these, Richard Boyd and colleagues provide a detailed overview of the basic thymic components, backed up by an extensive bibliography and full colour poster. The latest research trends are then summarized by Donald Palmer, Adrian Hayday and Michael Owen, followed by an examination of the mutual interdependence of thymic stroma and thymocyte by Mary Ritter and Richard Boyd.