The rapid evolution of microorganisms and cancer cells makes it difficult to treat tumours and infectious diseases, because resistance to drugs is the rule rather than the exception. Structures or models of protein–drug complexes help to understand how mutations lead to resistance and to design better drugs. However, it is difficult to reason how small changes in the structure lead to drug resistance. Thus, protein and drug dynamics need to be considered. Strategies to increase drug residence are sought after to increase the efficacy of drugs. Computational methods to calculate the effect of mutations on drug binding and residence times are being developed and improved, but are challenging. A priori prediction of a mutation's effect on drug binding is an even greater challenge. On the other hand, knowledge about protein–drug complexes has led to the development of multiple design strategies that aim to reduce mutation-driven drug resistance.