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Much of Earth’s magma is stored as extensive crystal mush systems, yet the prevalence of physical processes operating within mushes and their importance in volcanically active regions remain enigmatic. In this Review, we explore the physical properties and key processes of crystal mush systems. The initiation, evolution and decline of volcanic systems, modulated by heat supply and loss, could generate differences in the prevalence of mush processes through space and time. Additionally, regional tectonics alter mush properties, with mushes in cool wet settings having persistent residual melt, permitting more effective melt segregation than in hot dry settings. Disaggregation of mushes results in crystal mush material being mobilized or entrained into lavas and erupted, presenting opportunities to define the timescales and chemistry of some mush processes in volcanically active regions. Mush systems can be observed on length scales ranging from kilometres (using geological mapping) to micrometres (using crystal textures). Therefore, it is difficult to integrate data and interpretations across different fields. Improved integration of thermodynamics, textural analysis, geochemistry, modelling and experiments, alongside inputs from adjacent fields such as porous media dynamics, engineering and metallurgy will help to advance understanding of mush systems and ultimately improve hazard evaluation at active and dormant volcanic systems.