The amount of solar energy reaching the Earth in one hour equals the total amount of primary energy used in the world during one year. Thanks to the different solar technologies presently available, most energy needs of buildings can today be covered by solar energy in its various forms. Among mature solar technologies, Among mature solar technologies, active solar thermal (ST) systems for domestic hot water production (DHW) and space heating merit 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. Then why is this technology still largely underexploited when compared to photovoltaics (PV)? One reason seems to be that solar thermal systems are perceived as an architecturally unattractive option, while PV is seen as an appealing choice, with products well adapted to building integration, thanks to the thinness and dimensional flexibility of its modules, the choice of colour/texture, the small cables, and its light weight. Moreover, dimensioning and positioning a PV installation is easy, as electricity production can be independent of building needs or storage capacity, while this is less the case for solar thermal. Fortunately, progresses are being made in both these domains. Collector manufacturers are becoming aware of the shortcomings of their present products and work on new ones, and new simplified tools promise to assist architects in the complex dialog between architectural integration and system dimensioning.