Cellulose is the chief constituent of the plant cell wall and therefore is the most abundant biopolymer on Earth. However, cellulose synthesis is not limited to the plant kingdom: it is also found in a wide variety of bacteria, as well as in oomycetes, algae, slime mold, and urochordates, which are the only animals that synthesize cellulose. Nevertheless, cellulose synthesis has been mainly studied in plants and bacteria. In plants, cellulose confers mechanical support and protection against environmental stresses, and guides anisotropic cell growth. In bacteria, cellulose secretion is associated with biofilm formation, which protects cells from stresses or host immune responses and allows for community synergy in colonizing surfaces and capturing nutrients. In the context of our society, cellulose is an important part of woody plant biomass and is thus a renewable resource crucial for many industries, whereas bacterial cellulose is used for a plethora of biomedical and bioengineering applications. In addition, biofilms can reduce the susceptibility of bacteria to antibacterial agents and thus increase infection risk; understanding the molecular mechanism behind cellulose synthesis and biofilm formation is therefore of prime importance.In this primer, we aim to highlight the main differences as well as the common features of the molecular mechanism shared by the many species synthesizing cellulose across kingdoms.