The action of natural selection results in organisms that are good at survi
ving and reproducing. We show how this intuitive idea can be given a formal
definition in terms of fitness and reproductive value. An optimal strategy
maximizes fitness, and reproductive value provides a common currency for c
omparing different actions. We provide a broad review of models and methods
that have been used in this area, stressing the conceptual issues and expo
sing the logic of evolutionary explanations.