This book chapter discuss the world of Organ-on-a-Chip (OOAC) technology, uncovering its potential in revolutionizing preclinical research. The discussion begins by clarifying the fundamental principles of OOAC, emphasizing how cell lines integrate with microengineering, microfluidic technologies, and biomimetic principles to shape these innovative systems. Moving beyond the basics, the chapter focuses on standout projects that demonstrate the versatility of OOAC models across various biological scenarios. It addresses challenges in implementing OOAC technologies, providing insights into their current state and avenues for improvement. OOAC technology emerges as a promising solution to the failure of animal models, faithfully replicating organ-level functions and responses. The chapter explores how OOAC systems model diseases, study interactions within the body, and mimic human responses to drugs and pathogens in different organs. Despite challenges in gaining acceptance, recent advances in OOAC are highlighted, bringing us closer to the day when these models replace animal testing and become integral to drug development and personalized medicine.