Assembly is a capstone stage in the manufacturing process, which directly affects the quality of the final product. The rapid advancement of Artificial Intelligence (AI) technology has promoted the advancement of intelligent robotic assembly system. When combined with advanced sensing system and control strategy, AI can significantly augment the capabilities of assembly robots in all the phases, including perception, decision-making, and control. This chapter presents a brief introduction of assembly and the reasons why PiH assembly is chosen as the main topic in this monograph. This chapter also includes preliminaries and foundations of PiH assembly and reviews existing model-based and learning-based assembly methods.