The aim of this presentation is to give you insight into how biological invasions of parasites can be used as a tool to study host-parasite coevolution in nature. In this lecture, I will give a short introduction into parasite invasions, and introduce the two invasive parasites I work with: Mytilicola intestinalis and Mytilicola orientalis. Both parasites invaded the Eastern Atlantic and infect blue mussels Mytilus edulis. First the specialist M. intestinalis invaded from the Mediterranean Sea and later the generalist M. orientalis invaded from Japan. I will present data from a crossed coinfection experiment on phenotypic level, answering the questions “Do parasites evolve after invasion?” and “Is there competition between parasite species?”