This review describes the properties of block copolymer micelles which influence their efficiency as drug delivery vehicles for hydrophobic drugs. The key performance related properties wt discuss are loading capacity, release kinetics, circulation time, biodistribution, size, size distribution and stability. Each of the properties is discussed in detail with specific attention given to the way in which they may be changed or controlled, the aim being to allow the reader to tailor-make block copolymer micelles for a particular application. In addition, the last section of the review focuses on the morphology of the micelles as another performance related property which, to this point, remains unexplored in this connection. (C) 1999 Elsevier Science B.V. All rights reserved.