Hugo Mnsterberg (1863-1916) was a German-American psychologist. He was one of the pioneers in applied psychology, extending his research and theories to Industrial / Organizational (I/O), legal, medical, clinical, educational and business settings. He encountered immense turmoil with the outbreak of the First World War. Torn between his loyalty to America and his homeland, he often defended Germany’s actions, attracting criticism. He believed that mental illness had a psychological basis and made diagnoses based on behavioral observations, an interview and answers received by the patients whom he interviewed. These studies led him to publish the book, Psychotherapy (1909). In 1908, he published his controversial book, On the Witness Stand (1908), which talked about psychological factors that can affect a trial’s outcome. In 1913, he wrote the book Psychology and Industrial Efficiency which looked at problems with monotony, attention and fatigue, physical and social influences on the working power, the effects of advertising and the future development of economic psychology. His other works include: Psychology and Social Sanity (1914), The War and America (1914) and The Photoplay: A Psychological Study (1916).