The planetary boundary layer (PBL) is also known as the atmospheric boundary layer (ABL). It is the lowest part of the atmosphere and its behavior is directly influenced by its contact with the ground. It responds to surface forcings in a timescale of an hour or less. In this layer physical quantities such as flow velocity, temperature, moisture etc., display rapid fluctuations (turbulence) and vertical mixing is strong. Physical laws and equations of motions, which govern the planetary boundary layer dynamics and microphysics, are strongly non-linear and considerably influenced by properties of the earth's surface and evolution of the processes in the free atmosphere. Perhaps the most important processes, which are critically dependent on the correct representation of the PBL in the atmosperic models, are turbulent transport of moisture and pollutants. Clouds in the boundary layer influence trade winds, the hydrological cycle, and energy exchange.