Alginates are linear polysaccharides Alginates are linear polysaccharides found in brown seaweed and in certain bacterial species. The structure, hence the linear sequence of the two monomers β-D-mannuronic acid (M) and α-L-guluronic acid (G), and molecular weight determines alginate properties. Alginate forms gels in the presence of divalent ions under physiological conditions, rendering alginate an attractive material for cell encapsulation and tissue engineering applications. Alginate structure, hence the blocks of neighboring M (M-blocks), G (G-blocks) or alternating M and G (MG-blocks) can be tailored by the use of mannuronan C-5 epimerases that converts M to G in the polymer chain. The epimerases give different products as some produce polyalternating structures (MG-blocks) and some G-blocks of different length. As the G-blocks are mainly responsible for the gel formation with divalent ions, the introduction of G-blocks leads to alginate with better gelling abilities. Hence, alginate gel properties can largely be tailored by the use of epimerases. Upon chemical modification, the G- and M- are both modified. We have used oxidation and subsequent reductive amination for the grafting of alginates. In biomedical applications, the grafting of alginates with bioactive molecules can be attractive strategies to combine functional properties. In tissue engineering, grafting of alginate with peptides for cell adhesion is relevant to accommodate cells. We have also grafted cyclodextrin for drug release. As chemical modification in general disrupts the G-blocks and hence alginate gel forming abilities, chemical modification with subsequent epimerization is a strategy to control mechanical properties of grafted material. Alginate gel properties in cell encapsulation and tissue engineering applications will be discussed.