The biological world is full of phenomena that seem to run counter to Darwin's insight that natural selection can lead to the appearance of design. For instance, why do organisms in some species divide reproductive labor? The existence of non-reproducing organisms in such 'eusocial' species looks to be at odds with an evolutionary theory which posits traits exist because they help organisms survive and reproduce. What is the evolutionary advantage of an insect being distasteful to its predators? The distastefulness appears designed to deter predators, but can only affect the predator's actions when the insect is eaten; it is hard to see how such a trait could be passed on. This Element will cover the shared foundations of evolutionary explanations for these and other seemingly puzzling phenomena, focusing on the concepts of inclusive fitness and kin selection.