## 1 Introduction  
Worldwide population is increasing. According to different studies, this number has raised from six to seven billion between 1999 and 2011, and it is expected to further increase to nine billion by 2050 [1, 2]. This increasing trend has resulted in an increased demand for food supplies, with a strong impact on agricultural practices. On the one hand, the increase in the global population results in a higher demand for food products, requiring higher agricultural productivities [1, 2, 3, 4]. On the other hand, the social and economic activities developed by this increasing population have contributed to [1]: (i) a reduction in the area available for the production of food crops; (ii) water resources degradation and scarcity; (iii) accumulation of xenobiotic compounds in the soils; and (iv) degradation of soils' quality and fertility. To overcome the challenges related to the increase in global population and anthropogenic activities and meet the requirements for food supplies, an improvement in the productivity and sustainability of agricultural practices is required [1, 5]. Higher productivities can be achieved in different ways: (i) increasing crops' yields; or (ii) preventing culture losses under stress conditions (both biotic and abiotic).  
Chemically-based products have been widely used in agriculture, either as plant-growth promotors (improving crops' yields), or as plant protection agents (protecting plants from different stress conditions) [5, 6, 7, 8]. However, most of these agents are toxic and their accumulation in the plants and in the soil can be a threat to humans and to the environment. Moreover, their accumulation has been associated with the development of microbial resistance to several drugs [3; 5; 7; 8]. Considering the negative impacts of these products, current regulations are limiting the use of mineral fertilisers and chemical products in agriculture [5; 7]. In the search for more sustainable and environmentally-friendly solutions to improve agricultural productivities, researchers have focused their attentions on biologically-based products, with microalgae and cyanobacteria emerging as a valuable resource for crops' production and protection due to their biofertilising and biostimulating potential [1; 2; 3; 5; 7; 9].  
The use of algae in agriculture dates from thousands of years ago [5; 9]: for example, in coastal areas of Europe, farmers used to apply algae harvested near the shore in their cultures, both directly or after composing, observing positive effects in soil fertilisation. Since this period, algal biomass has been extensively used in agriculture, but in the 20 th century, products obtained from algal extracts have attracted the attention of farmers worldwide [5]. In fact, a wide variety of biologically-active compounds extracted from algae and cyanobacteria (e.g., phenols, terpenoids, free fatty acids--FFAs, polysaccharides, and carotenoids) has demonstrated to have promising effects in crops' production [2; 7]. According to the literature, algal metabolites can play an important role in [4; 9]: (i) soil decontamination and fertilisation; (ii) plant protection against biotic and abiotic stress factors; and (iii) plant development. In addition, microalgae and cyanobacteria also present phytohormones, which are known for their activity as plant-growth promotors [7; 10; 11]. Taking into account their potential benefits for the development of a sustainable agriculture, both biomass and extracts from microalgae and cyanobacteria are commercially available [9].  
This manuscript presents an overview of how algal biomass and extracts can improve crops' production, presenting the most recent applications of these microorganisms in the agricultural sector.