The integration of photonics and electronics has transformed the landscape of modern technology. At the forefront of this revolution is the development of Photonic Integrated Circuits (PICs). Historically rooted in the traditional 2-D fabrication processes inherited from electronic Integrated Circuits, PICs shifted to 3-D configurations, introducing new design philosophies that impact scalability, efficiency, and performance. This convergence of electronic and photonic circuits presents unique challenges and great opportunities. This book provides an introduction to photonic integrated circuits and the transition from 2D to 3D PICs. It then describes design and fabrication techniques of 3D PICs and related challenges and solutions. Finally, applications of 3D photonics, emerging technologies and industry outlook are also discussed.