Sexuality encompasses diverse behaviours and meanings that are shaped by several factors. Sexual reproduction in invertebrate and vertebrate animals is much more prevalent than asexual reproduction. Even prokaryotes and viruses show "parasexual reproduction". An equally challenging task is to understand the evolutionary logic behind the diverse courtship and mating strategies and phenotypic plasticity of sex determination. In human, although sexual activity has, until very recently, been essential to reproduction, this did not preclude the non-reproductive importance of sexual relationships and non-conceptive copulations. Why are there only two kinds of gametes in most eukaryotes? Why are few animals and most flowering plants hermaphroditic? Why do male animals compete more for access to females than the other way around in most animals that have been studied all over the globe? This book presents more questions than answers. Hence different aspects of sex, sexual reproduction have been discussed so that it may prove useful to biologists and those with a genuine interest in the key issues of evolution of sexuality and provide an avenue for them to understand it.