This book provides a comprehensive overview of the principles and mechanisms underlying animal and plant development. It covers key processes such as fertilization, cell differentiation, morphogenesis, and organogenesis. The book integrates genetic, molecular, and cellular approaches to study development, highlighting model organisms and experimental techniques. Designed for undergraduate and graduate students, it aims to connect developmental biology with other disciplines such as genetics and evolution. Supplementary materials include problem sets, lab exercises, and access to online animations and simulations.