Among mature solar technologies, active solar thermal (ST) systems for domestic hot water production (DHW) and space heating deserve to play a key role. Their proven high efficiency, low cost and short payback time naturally make them a prime choice for energy efficient buildings. The term “active solar thermal” encompasses several technologies, characterized by different efficiencies and application possibilities. Here, we will consider only the systems relevant for DHW and space heating in buildings: Glazed flat plate collectors, Unglazed flat plate collectors, Evacuated tubes collectors.