The magnetic properties of many magnetic materials can be controlled by external stimuli. The principal focus here is on the thermal, photochemical, electrochemical, and chemical control of phase transitions that involve changes in magnetization. The molecular compounds described herein range from metal complexes, through pure organic compounds to composite materials. Most of the Review is devoted to the properties of valence‐tautomeric compounds, molecular magnets, and spin‐crossover complexes, which could find future application in memory devices or optical switches. Under control: The behavior of certain magnetic materials can be controlled by external stimuli such as temperature, light, and pressure. Typical examples of such switchable materials are valence‐tautomeric compounds, molecular photomagnets, and spin‐crossover complexes, which could find application in memory devices or optical switches.