Biostimulants improve growth of plants by enhancing nutrient uptake efficiency. This chapter presents classification and legal aspects of biostimulants in terms of registration. Seaweed extracts improve abiotic stress tolerance of horticultural and agronomic crops. The mechanism of seeweed extracts action is shown with the special role on the effect of primary and secondary metabolites acting as growth stimulating and protecting factors, as well as antibacterial agents. Additionally, those preparations modulate growth rhizosphere microbial population. The mode of action is also related with plants physiology, in particular germination of seeds, growth of shoots, roots, fruit set. Biostimulants also have the beneficial influence of quality of food crops, which is of particular importance to the farmers. Further research is needed to confirm the effectiveness of biostimulatnts on plants and scientifically prove the mechanisms of action of these products.