The aim of this study was to investigate the method Henri Cartier-Bresson used when he took his photographs and also what he meant when he talked about the decisive moment. The investigation was based mainly on some of Cartier-Bressons photographs and also literature written by Cartier-Bresson and others. Some interviews and conversations with Cartier-Bresson was also investigated. Theories on documentary photography and the concept of previsualizing was used and formal analysis and semiotic analysis was used when the photographs were analysed. The investigation shows that Cartier-Bresson always wanted the geometry to be perfect and in balance. When that moment occurred it was the decisive moment.