Written primarily for 16-19 year old students, this primer aims to extend students' knowledge and inspire them to take their school-level learning further. It explores topics that are familiar from the curriculum and also introduces new ideas, giving students a first taste of the study of biology beyond school-level and demonstrating how concepts frequently encountered at school are relevant to and applied in current research. This is the ideal text to support students who are considering making the transition from studying biology at school to university. This primer introduces students to key elements of the endocrine system, including the structure and function of the major types of hormones. It explores the role of hormones in disease, in development, in reproduction and in behaviour. Although humans are its main focus, it draws on illuminating examples from a range of other species, including sex changes and aggression in fish, metamorphosis in amphibians, induction of ovulation in different species, and bonding behaviour in birds. The final section of the book looks at ways in which hormones can be used in medicine and agriculture, and can be abused to enhance sporting performance and weight loss. This primer will expand the breadth and depth of student understanding of the endocrine system, its variety, and its huge importance.