Microbial polysaccharides are used in food, pharmaceutical, and medical applications: this wide range of usefulness derives from the great diversity in structural and functional properties. We consider the structure, properties, extraction, production, modification, and applications of a selected range of polysaccharides. Xanthan (Xanthomonas campestris) and xylinan (Acetobacter xylinum) have a β(1→4)-linked glucan main chain with alternating residues substituted on the 3-position with a trisaccharide (xanthan) or pentasaccharide (xylinan) and are used commercially as viscosifiers. Gellan (Pseudomonas elodea) and curdlan (Alcaligenes faecalis) are xanthan-like polysaccharides although their application as a thickening agent is limited to Japan. Exopolysaccharides from Cyanobacteria (blue-green algae) have been reported to have potential applications as emulsifiers, viscosifiers, and bioflocculants. Pullulan (Aureobasidium pullulans) and dextran (Leuconostoc mesenteroides) are α(1→6) d-glucopyranosyl polymers with (1→2), (1→3), or (1→4) branches. Both the polymers are used as gel filtration calibration materials. Bacteria can also produce alginate and hyaluronic acid (HA) for use in applications such as viscosurgery and viscosupplementation. Two structurally similar fungal polysaccharides scleroglucan and schizophyllan used in cosmetics and pesticides are also discussed. Finally, purified extracts of polysaccharides from pathogenic bacteria (e.g., Streptococcus pneumoniae) may themselves be immunogenic and can be used at least in principle to produce immunity against the organism that is producing them. As a result, vaccines against some of these organisms are now available.