After more than a century of intensive research, sex allocation patterns are now remarkably well understood, and this success of sex allocation theory is often hailed as a triumph of evolutionary theory. As demonstrated by Stuart West, demographic and genetic parameters are now routinely derived from sex ratio patterns, by means of an 'inverse optimality' approach. However, the success of sex allocation theory seems to be largely restricted to invertebrates, partly because standard assumptions of the theory are systematically violated in vertebrates (Ben Sheldon). To bridge the gap between data and theory and in order to make sex allocation theory more widely applicable, Ido Pen systematically extended the theory to include 'sex allocation in a life history context'. During the symposium, Ido defended his PhD thesis on this topic (see http:// www.biol.rug.nl/theobio). In his talk and in his thesis, Ido demonstrated convincingly that, in spite of the inherent complexity of the problem, promising progress can be made with the help of a generalized reproductive value approach.