Ocean stratification describes the layering of seawater, with lighter (warmer, fresher) water overlying denser (cooler, saltier) water. It plays a fundamental role in regulating the vertical exchange of heat, carbon, oxygen, and nutrients in the ocean, with important consequences for marine ecosystems, biogeochemical cycles, and climate. No single metric is appropriate for diagnosing ocean stratification across all applications. Different metrics may be better suited to different scientific question, spatial and temporal scales, and available datasets. As estimates of stratification change can be sensitive to the metric used, careful consideration of metric choice and comparability is essential. While ocean stratification is widely observed to have increased globally over recent decades, and is projected to continue increasing in the future, important questions remain regarding the spatial and temporal heterogeneity of these changes, their underlying controls, and their consequences. Improved quantification, continued development and evaluation of ocean models, and attribution of observed and simulated changes to key mechanisms are needed at both global and regional scales. Changes in ocean stratification can affect ocean deoxygenation, primary production and marine biomass, the carbon cycle, ocean circulation and overturning, ocean heat uptake, tropical cyclones, marine heatwaves, and ultimately human societies. Understanding these links is particularly important for assessing how changes in the ocean may feed back on the climate system. This session welcomes contributions from observers and modellers addressing ocean stratification over a wide range of spatial and temporal scales, including its measurement, variability, drivers, trends, modelling, and impacts. Interdisciplinary subject are particularly encouraged.