<p>In evolutionary terms, chemical senses are the oldest sensory modality and play key roles in ecological functions such as orientation, foraging, and socio-sexual communication. Chemical perception encompasses olfaction, taste, and chemesthesis, and has been widely studied across vertebrates. However, these senses remain poorly documented in marine mammals, particularly cetaceans. The transition from land to water profoundly reshaped cetacean sensory systems, and for decades they were considered incapable of chemoreception, implying a complete regression. Recent studies challenge this view, suggesting a more complex and incomplete regression. In this review, we synthesize current knowledge from different perspectives, using an integrative framework. First, we examine the anatomical basis of chemoreception, identifying relevant structures enabling this process and their development across early life stages. Second, we explore genetic and molecular foundations, focusing on genes underlying the development and function of chemical receptors. Third, we review behavioural evidence, highlighting experimental studies on cetacean responses to chemical cues in ecological contexts such as predator–prey interactions, foraging, and social behaviour. This approach allows us to characterize and quantify chemosensory regression in cetaceans and confirms divergent evolutionary trajectories between odontocetes and mysticetes. We also identify key unresolved questions: (1) cetaceans appear to use social chemical signals, but the mechanisms remain unknown; (2) reliance on taste is still debated, with conflicting findings; (3) the ontogeny of chemical senses remains poorly understood; and (4) alternative chemical modalities such as chemesthesis have been largely overlooked despite hypothesis on their potential ecological importance.</p>