Includes bibliographical references.

1 What Was Reductionism?
1.1 Reductionism in Twentieth-Century Philosophy of Science
1.2 The Philosophy of Biology Takes Off
1.3 Twenty-First-Century Antireductionism
2 Biology as Natural History
2.1 Reductionism in a Historical Science
2.2 Developmental Biology and Explanatory Reduction
2.3 Reductionism's Hostage to Fortune
3 Reductionism and Natural Selection
3.1 Showing that Physics Is Enough for Adaptation by Natural Selection
3.2 Showing How Physics Makes Natural Selection the Only Way Adaptations Can Arise
3.3 Physical Asymmetry and Adaptational Evolution
4 Reductionism Makes Way for Mechanism
4.1 How Mechanisms Explain
4.2 Mechanism and Higher-Level Explanation
4.3 Mechanism and Functional Individuation in the Life Sciences

This Element explores the debate surrounding reductionism in biology, tracing its development from the post-positivist era to the late 20th-century discussions on supervenience, multiple realizability, and explanatory exclusion. It illustrates how the more widely accepted 21st-century doctrine of "mechanism"—often described as "reductionism with a human face"—inherits both the strengths and challenges of the perspective it has largely replaced.