This chapter introduces the most common passive strategies that can be used to heat a building. The simplest method to passively heat a building is to increase solar gains to the interior of the building. A passive solar system will capture heat during the day coming from solar radiation and will store it for night use. In cold climates, a building will lose heat by conduction through its walls, roof, and floor; by convection through the window frames; and by radiation through sky-facing surfaces. Losses by conduction can be reduced by improving the insulation of the building skin, creating a more compact building, and making a tighter envelope. In an east—west-oriented building, a polar-facing exterior wall will receive no direct solar radiation during the winter and becomes major source of heat loss. Passive heating systems are usually classified in three main groups: direct gain, indirect gain, and isolated gain systems.