Islands have fascinated biologists since the days of Charles Darwin and Alfred Russel Wallace and before, providing the inspiration for substantial theoretical development that has advanced our understanding of global biodiversity patterns and the mechanisms that underpin them. As such, they are often termed ‘natural laboratories’, providing the ideal setting to study the interface between ecology, evolution and conservation. Part of this fascination no-doubt arises from islands harboring a disproportionate amount of global biodiversity given the amount of land-mass they occupy (roughly 15–20% of global terrestrial species present in just 3.5% of global land), including large numbers of endemic forms not found anywhere else. Interestingly, more than 25% of human languages, many of which are also threatened with extinction, are also to be found on islands. In this primer, we provide an overview of the field of island biogeography, splitting it into three main sections. First, we explore some of the reasons that make islands, and the species that have evolved on them, unique and scientifically rewarding study systems for ecologists and biogeographers. Second, we delve into the key island biogeography works in order to provide an introductory summary of some of the main theoretical models developed to explain species diversity patterns on islands. Unfortunately, as well as representing captivating environments to study, islands are also highly threatened systems. As such, we end with an overview of the drivers and impacts of anthropogenic environmental change on islands, providing examples of some of the extraordinary island species that humans have driven extinct.