Large-scale assessments of threats to salmonids have increased in recent decades; yet the role of crayfish—both native and nonnative—has received comparatively limited attention, and existing knowledge remains fragmented across the literature. This study addresses this gap by synthesizing current evidence of crayfish effects on salmonids across ontogenetic stages and identifying critical knowledge gaps to guide future research. The state of published knowledge on the impact of crayfish on salmonids is relatively disparate across taxonomic groups, geographic regions, and environmental contexts. Our review identified 35 articles (comprising 60 crayfish-salmonid study outcomes) published over the past half-century, involving 10 crayfish species and 12 salmonid species. Half of all study outcomes explored the salmonid egg stage (51%), alevin (8%), fry (10%), juveniles (14%), adults (12%), and post-mortality carcasses (5%). Salmonids experience substantial mortality during early life stages, particularly from egg deposition through fry emergence. Salmonid eggs are vulnerable to predation by both native and nonnative crayfish, which act as active foragers as well as scavengers. Although laboratory studies have provided valuable insights, their applicability to natural systems remains limited. Key knowledge gaps persist, particularly regarding the functional feeding responses of crayfish (i.e., egg consumption as a function of availability) and the interactions between environmental and biological factors that shape per capita effects on salmonids. Crayfish–salmonid interactions extend to fry and juvenile stages through both direct predation and competition for shelter and invertebrate prey, which may constrain salmonid growth. Despite these plausible mechanisms, empirical studies are very scarce. Evidence for effects on adult salmonids is even more limited, with correlative studies yielding mixed results, and the role of crayfish in nutrient cycling via carcass scavenging remains poorly understood. By illuminating the complex, context-dependent nature of crayfish–salmonid interactions, we propose 10 priority research directions to inform future management and conservation actions.