Wildlife in Britain has been a source of delight for many generations of people. It has also been the inspiration for writers and poets, and has become the subject of serious scientific study. Media coverage has increased steadily, promoting wide interest in animals and plants in the UK. However, there are both costs and benefits arising from wildlife. Damage, including to crops and trees, by a range of ‘pest’ species can be extensive, and conservation measures such as maintenance of nature reserves are also costly. Nevertheless, these downsides are more than outweighed by the benefits to people and the economy. A wide range of ecosystem services make use of wildlife and healthy ecosystems, and access to these outdoor delights is highly valuable in the promotion of mental health. Unfortunately, more than half of Britain’s wildlife has been in continuous decline for several decades. Proximal reasons for this sad fact are summarised before considering them, and the primary issue of human numbers, in subsequent chapters.