Most marine animals have a biphasic life history including a pelagic larval phase. In demersal taxa, most dispersal , a critical part of connectivity, occurs during the larval phase. Understanding the scale of larval dispersal is fundamental to study of ecology and evolution of demersal marine species and their management. The difficulty of studying movement of small larvae over days to months in the ocean, means biophysical models are commonly used to estimate dispersal. Although the physical parts of such models are generally well-based and often validated, the same cannot be said of the biological parts, in spite of explicit published advice on their construction since 2009. Research since the 1990s shows that many decapod crustaceans and teleost fishes have well-developed swimming, orientation and sensory abilities that can strongly influence dispersal outcomes, and that some of these are possessed by other taxa. This has overturned the ‘simplifying assumption’ of passively dispersing larvae in all taxa. Yet, few contemporary models embrace this understanding by incorporating larval behaviour (or in some cases, even adult behaviour). This talk reviews the history of increasing knowledge of larval-fish behaviour, methods to obtain and incorporate this into models and the underappreciated need to validate models.