Hallucinations can occur in different sensory modalities within an individual, both simultaneously and serially in time. Historically, they have typically been studied in clinical populations as phenomena occurring in a single sensory modality. Yet, hallucinatory experiences that occur in two or more sensory systems - multimodal hallucinations (MMHs) - are more prevalent than previously thought, and may have greater adverse impact than unimodal ones, but they remain relatively under-researched. Why people experience multi- modal hallucinations, what the implications for the person are, and how this could impact how such experiences are treated when distressing remain under-studied questions. Here, we review the available literature on MMHs and discuss some key concepts in the field, namely a) the definition and categorisation of both serial and simultaneous MMHs, b) which assessment tools are available and how they can be improved, and c) the explanatory power that current hallucination theories might have for MMHs. Overall, we suggest that current models need to be updated or developed in order to account for MMHs and to inform research into the underlying processes of such hallucinatory phenomena. We make recommendations for future research and clinical practice, highlighting the potential clinical impact of MMHs, the need for better assessment tools that can reliably measure MMHs and distinguish them from other related phenomena (such as delirium), as well as the need for service-user involvement in the validation of classification systems of MMHs.