Many animals use dazzling colours and body patterns to attract mates, deter predators and compete for resources. In particular, aposematic signals are conspicuous color patterns used by animals to promote avoidance learning in predators. These eye-catching color displays are frequently associated with strong chemical defences, but how such defences and conspicuous displays coevolve is frequently unclear.  Here, we present comparative data on colour patterns and chemical defences in a wide range of nudibranchs, which are soft-bodied gastropod molluscs. We quantified colour patterns from a potential predators perspective using visual modeling and pattern analysis, and also conducted a systematic chemical survey of secondary metabolites from a range of nudibranch species. We then examined the relative strength of these chemical defences against a range of predators. To date, we have found evidence that aposematic nudibranchs have evolved from cryptic, relatively undefended species; however, species that have the strongest visual signals do not necessarily have the strongest chemical defences. Empirical data on the relationship between conspicuousness and the strength of chemical defences in a wide range of species has enabled use to explore key evolutionary hypotheses on the evolution of aposematism.