Only a small proportion of DNA intercalators known today exhibit experimental anti-tumour activity and even fewer are useful clinically, raising the question of precisely what properties are necessary for an anti-cancer effect. A search for answers to this question has revealed a complex picture of how intercalators can interact with DNA and other macromolecules, which is the subject of this review. A description of some of the main intercalating anti-tumour agents is followed by a discussion of the principles of non-covalent drug-DNA interactions. The possible mechanisms by which DNA intercalation leads to anti-tumour activity are described and the concept of ternary complexes involving DNA, drug and DNA-binding proteins is developed. DNA topoisomerase II, which constitutes a prime target for intercalating anti-tumour drugs, is discussed in relation to ternary complex formation, tumour selectivity and drug resistance. Factors affecting the transport of drugs into and within cells are also discussed. An emphasis is placed in this review on guiding principles which may facilitate the design of novel or more effective agents in the future.