Until today, flat-plate collectors have maintained their leading position in solar energy. Their principle of operation is quite simple: Sunlight falls onto an absorber which releases the energy as heat to a fluid. Modern flat-plate collectors also work according to the same principle, but they show important modifications. New types of material have higher absorption and lower emission levels; a glass cover generates a conducive greenhouse effect inside flat-plate collectors. An optimised insulation finally ensures that as little heat as possible is lost at the rear side. Although tube collectors have a higher energy yield, flat-plate collectors will continue to enjoy high popularity due to their compact design and high market acceptance.