Biostimulants are applied to crop plants in the aim of modifying plant physiological functions and of increasing crop productivity or quality. They may be regarded as some kind of ‘functional ingredients’ in plant nutrition, distinct from fertilizers and plant protectants. Despite the agricultural use of biostimulants such as seaweed extracts and humic acids since decades, it is only recently that the concept and the naming have been recognized by regulations on fertilizing products. Biostimulant products placed on the market are identified by claims, describing the intended effects on crops, such as improved nutrient use efficiency and tolerance to abiotic stress. Biostimulants fit the objectives of the sustainable intensification of crops, but their development calls for an improved knowledge of their bioactivity on plants and associated microorganisms, of the responsiveness of the recipient plants and environments, and of their interactions with fertilizers and other agricultural inputs.