Polynyas are nonlinear-shaped openings within the ice cover, ranging in size from 10 to 105 km2. Polynyas play an important climatic role. First, winter polynyas tend to warm the atmosphere, thus affecting atmospheric mesoscale motions. Second, ocean surface cooling and brine rejection during sea ice growth in polynyas lead to vertical mixing and convection, contributing to the transformation of intermediate and deep waters in the global ocean and the maintenance of the oceanic overturning circulation. Since 1990, there has been an upsurge in polynya observations and theoretical models for polynya formation and their impact on the biogeochemistry of the polar seas. This article reviews polynya research carried out in the last 2 decades, focusing on presenting a state-of-the-art picture of the physical interactions between polynyas and the atmosphere-sea ice-ocean system. Observational and modeling studies, the surface heat budget, and water mass transformation within these features are addressed.