For practical reasons, it is likely that low-temperature solar collectors have a more immediate future than high-temperature or photovoltaic generation of electricity. This paper discusses the physics of bare and covered flat-plate collectors. The greenhouse effect is the result of reducing convection to the point that radiation trapping becomes important. Nevertheless, at collector temperatures within 20-30 degrees C of ambient, convection from the collector surface is so important that a special absorber with low ir emissivity may be no more efficient than a good, black absorber. At higher temperatures, selective absorbers are desirable. In the low temperature range, collection efficiency can be kept well over 80%, but falls rapidly with increasing collector temperature. This suggests that solar power may see early application in conjunction with heat pumps for heating and air conditioning.