**Abstract**

Alginate is a linear anionic heteropolysaccharide with a chemical structure consisting of 1,4-linked subunits of β-D-mannuronic acid (M) and its C-5 epimer α-L-guluronic acid (G). It is well known that the monomer composition and molecular weight of alginates affect their properties and influence their use in the food and pharmaceutical industries. Alginate is usually extracted from seaweed for commercial purposes, but can also be produced by bacteria as exopolysaccharide (EPS). *Pseudomonas* spp. and *Azotobacter vinelandii* are well-known alginate-producing microorganisms. Their biochemical machinery for alginate biosynthesis is influenced by changing culture conditions and manipulating genes/proteins, making it relatively easy to obtain customized EPS with different molecular weights, M/G compositions, and thus physicochemical properties. Although these two genera have very similar biosynthetic pathways and molecular mechanisms for alginate production, with most of the genes involved being virtually identical, their regulation has been shown to be somewhat different. In this chapter, we present the main steps of alginate biosynthesis in bacteria, including precursor synthesis, polymerization, periplasmic modifications, transport/secretion, and post-secretion modification.

**Keywords:** Exopolysaccharide, *Pseudomonas* spp., *Azotobacter vinelandii*, alginic acid, mannuronic acid
