Interspecific interactions have several evolutionary consequences: for example, two species may compete, hybridize, or behave aggressively towards each other, or there may be predator–prey interactions. One consequence of these interactions is the evolution of premating reproductive isolation between the two species. The most obvious interspecific interaction, which has an effect on reproductive isolation, is the avoidance of hybridization, or, in other words, the reinforcement process. The theory of reinforcement states that when hybridization is maladaptive, selection pressure causes a divergence in female mate preference and/or in male secondary sexual characters. It is often assumed that, ultimately, females are responsible for hybridization because they are the choosier sex and their co-operation is needed for successful copulations. Despite this view, it is possible that males, rather than females, are responsible for hybridization, especially in species in which males can force copulations. In addition to the avoidance of maladaptive hybridization, other interspecific interactions, such as aggression or predation, may also have an effect on premating reproductive isolation. For example, if interspecific aggression is directed towards males with the most exaggerated sexual characters because of the similarity of these characters between the two species, natural selection for sexual character divergence may arise. Just like the reinforcement process, this process may lead to a strengthening of premating reproductive isolation. In this chapter, we will review the effect of interspecific interactions on premating reproductive isolation. As examples we will mostly use the studies conducted on Calopteryx damselfies.