Harmful algal blooms (HABs) are the cause of important damages to marine living resources and human beings. HABs are generated by micro-algae. These marine species are primarily introduced through ballast water of ships and, to a lesser extent, through import of living fish, in particular shellfish. Effective and efficient regulation of HABs requires an integration of insights from biological, economic and legal sciences. Such an integration consists of (a) a clear identification of the bio-ecological pathways and overall consequences related to the damages of HABs; (b) an assessment of monetary costs of HABs; and (c) an understanding of the set of complementary legal-institutional and economic instruments dealing with HABs through prevention, restoration and amelioration. This paper discusses each element in detail, in which biological, economic and legal aspects come together, drawing conclusions for decision making in marine management. In order to move away from the general level of discussion, an example of HABs is presented in which, biological, economic and legal aspects are combined. © 2002 Elsevier Science Ltd. All rights reserved.