Sexual reproduction is one of the most fundamental of biological processes: (i) the creation of gametes, (ii) their fusion and (iii) the formation of a viable embryo are all shaped by a major evolutionary force: post-copulatory sexual selection comprising sperm competition and cryptic female choice. This project will make substantial advances in all three areas, using birds (mainly zebra finch) as model organisms. (i) Sperm size and shape: A major hypothesis for the enormous variation across species in the design of male gametes is that a trade-off exists between sperm size and number. We will test this, by estimating (for the first time) the energetic costs of making sperm. (ii) Sperm-female and sperm-egg interactions. We will establish, how sperm from different males interact within a female and will do this, uniquely, by using transgenic zebra finches whose sperm flagella are labelled with green fluorescent protein (GFP). This allows us to distinguish (in the oviduct and in ova) the sperm from GFP- and normal males and to visualise how they interact to generate last male sperm precedence. (iii) The genetic and environmental causes of embryo mortality. We will explore the environmental effects of temperature on embryo development and survival and consider the special case of brood parasites that expose their ova to elevated temperatures through ¿internal incubation¿. We will explore the genetic effects of DNA integrity, aneuploidy and compatibility on embryo survival. Together, these three interconnected strands will revolutionise the study of reproduction, answering the most outstanding questions in the field through a combination of novel techniques and novel hypotheses.