In temperate and tropical marine ecosystems, symbiotic pinnotherid (‘pea’) crabs and palaemonid shrimps can be found living in association with a wide range of different invertebrate hosts. Some of these species live inside ascidians (sea squirts) and bivalve molluscs (clams, mussels), and are therefore labelled as ‘endosymbionts’. The aberrant lifestyle and ecology of these animals caused them to adapt morphologically over time. This is most evident in the shape of their bodies, which are generally smoother and larger than their relatives which live on their hosts’ surface. Moreover, various lineages have evolved ‘Velcro’-like hairs, hooks, and scales on their walking legs, presumably to have better grip inside their hosts. The similar habitat types caused various lineages within and between the two major crustacean groups to adapt in similar ways, showing signs of convergent morphological evolution. In this thesis, a combination of morphological and molecular data is used to find patterns of ecomorphological convergence within the symbiotic shrimps and pea crabs. Literature was reviewed to search for complete host records, new species and genera were described where needed, and molecular and morphological analyses were performed to place the species in the ‘tree of life’, and to showcase instances of convergence within these family trees.