Gustav Senn analyzed for the first time light-induced movement and arrangement of chloroplasts. Using many plant species he performed physiological analyses of chloroplast migration in response to external stimuli, with emphasis on light. He determined light paths within a cell by measuring refractive indices and optical thickness of cellular compartments and confirmed that chloroplasts migrate towards the region where the light intensity is optimum. After 6 to 7 years' concentrated study, Senn published the famous monograph “Die Gestalts- und Lageveränderung der Pflanzen- Chromatophoren” (The Changes in Shape and Position of Plant Chloroplasts) in 1908. This book has stimulated many plant physiologists and photobiologists, because Senn not only thoroughly classified and defined various types of light-induced chloroplast migration but also already described possible interaction of different photoreceptor systems in Mougeotia more than 50 years before the discovery of phytochrome. This book also contains still useful experimental hints and overlooked findings on the interaction between light and other factors, such as temperature, water content, and nourishment. After publishing this book, Senn retreated from the study of chloroplasts and became a researcher of the Greek philosopher, Theophrastus. In this review, I introduce his biographical background and then summarize some of his key research accomplishment.