Insects are a fascinatingly diverse and beautiful group of animals. They are found on all continents, in caves, underground, inside other insects, in rivers, lakes, puddles, and in our houses. To date, over a million insect species have been named. In this Very Short Introduction, Simon Leather explores insects' evolution, behaviour, and development, highlighting their pivotal role in supporting ecosystems across the planet. He considers the threats of environmental change, including climate change, to insects globally and the potentially catastrophic impact of insect population declines.