This book is an introduction to the diversity of structure and function in animals at the tissue and organ system level. The focus of this book is on principles and mechanisms that sustain life and maintain homeostasis, including water balance, gas exchange, acquisition and transport of oxygen and nutrients, temperature regulation, electrical and chemical signal transmission, sensory processing, and locomotion. The content in this open textbook was adapted from other open textbooks (cc by 4.0) resources or created/written by Sanja Hinic-Frlog and collaborators. Collaborators include: Jessica Hanley, Simone Laughton, and invited undergraduate and graduate students at the University of Toronto Mississauga.