Regeneration has captured human imagination for much of recorded history. Its sociological influence is evident in ancient and modern folklore, art, politics, and even language. In many ways, the study of regeneration helped establish the field of biology as a legitimate scientific discipline. Furthermore, regeneration research yielded critical insights that challenged flawed scientific models and uncovered fundamental principles underpinning the workings of life on this planet. This chapter details some ways in which the study of animal regeneration-with special emphasis on planarian regeneration-influenced the evolution of thought in biology. This includes contributions to the discovery of stem cells, the nature of heredity, and key concepts in pattern formation.