This chapter describes the fundamental evolutionary theories that seek to explain the presence of aging despite its apparent detrimental effects on individual fitness and explores key evidence and shortcomings of these theories. It focuses on the observed trade-offs between life span and reproduction, highlighting potential molecular mechanisms by which selection can fail to eliminate, or even promote, patterns of senescence. An underexplored avenue by which selection can act on aging, mate choice, and sexual selection is then discussed leading to the development of a verbal model whereby mate choice could promote senescence as a by-product of honest sexual signaling. The chapter then explores how the described evolutionary theories pertain to human diseases, and identifies the critical absence of some important evolutionary processes in the evolutionary theory of aging and disease. Finally, it provides an in-depth understanding of why species age, and implications on human aging.