The structures, origins, biogenesis, synthesis and bioactivity of a selection of N-heterocyclic marine alkaloids are reviewed. The emphasis is on compounds poised as potential anticancer drugs: lamellarins (pyrroles), cephalostatins/ritterazines (pyrazines) and ecteinascidins (isoquinolines). Also discussed are examples of bioactive marine alkaloids that have emerged as novel leads. These include manzamines (β-carbolines), variolins (pyridopyrrolopyrimidines) and the pyrroloquinoline family. This review emphasises the role of marine alkaloids as an important source of leads for drug discovery.