In an agroecological context (French Law on the Future of Agriculture), the reduction of herbicide uses become a crucial issue. It requires developing new technologies allowing a better knowledge of the field. The company AIRINOV, specialized in use of Unmanned Aerial Vehicle (UAV) dedicated to precision agriculture wants to explore a new approach by UAV to localize weed infestation areas. This work aims to develop a method analyzing images acquired by UAV to detect and map weeds in a field. The study is carried out on crops widely grown in France and requiring weed control at a young state of the plant : maize, sunflower and sugar beet. With its on-board sensors (GPS, multispectral camera…), the UAV can acquire georeferenced multispectral images of a whole field with a spatial resolution of 6 cm/pixel. The multispectral camera is also used to capture images at 6 mm/pixel on small field parts to assess the effect of image resolution. . A weed detection method is specifically developed to study multispectral image acquired by UAV. This method uses a spatial analysis based on the Hough Transform to localize crop rows. Weeds are then identified as the vegetation located in the inter-row space, and infestation maps are deduced from their GPS coordinates. Afterwards, these maps are finally transformed into herbicide application maps for site-specific weed management.