Menno Reindert Kruk was born in 1944 in the Netherlands. He did his PhD on the ‘Origins of Hypothalamic Aggression’ and became President of the International Society for the Study of Aggression. He coordinated Hungarian-Dutch cooperation on ‘Stress and Aggression Interactions’ and an interdisciplinary multicenter research program on the ‘Neuroanatomy and Temporal Structure of Hypothalamic Responses’. His main research focus was the interaction between brain function, hormones, and behavior, with the aim of understanding brain mechanisms during violent behavior. He specifically explored how aggression could be studied using methods from the natural sciences and animal models, first to clarify which hypothalamic neurons mediate attack during electrical stimulation and then to register their activity during social interactions in order to understand ‘pathological’ processes in aggression. The research methods he used included ethological, pharmacological, endocrine, physiological, and mathematical approaches. He developed animal models of functional and pathological aggression and the mathematical tools to describe and analyze the effects of drugs and hormones on behavioral structure and on social interactions between animals. He studied the crucial role of corticosteroid feedback to the brain for the control of aggressive behavior and studied the interactions between the processing of conflict-related stimuli and stress hormones in humans.