This review covers the isolation, structure determination, synthesis, chemical transformations and biological activity of indolizidine and quinolizidine alkaloids from microbial, plant and animal sources. Included in the review are the hydroxylated indolizidines lentiginosine, swainsonine, castanospermine and their analogues; alkaloids from animal sources, including ants, amphibians and beetles; ipalbidine, phenanthroindolizidines and related alkaloids; Lycopodium alkaloids; lupine alkaloids; and alkaloids from bacterial and marine sources. The literature from July 2002 to June 2003 is reviewed, and 174 references are cited.