Solar cells made from organic semiconductors are very attractive because they can potentially be made at very low cost in roll-to-roll coating machines. This tutorial will start with an introduction to organic semiconductors that will explain why conjugated molecules can be semiconductors, what determines their absorption spectrum, how molecular packing affects the charge carrier mobility and how they are used to make transistors, light-emitting diodes and solar cells. Then the various designs that have been used to make organic solar cells- single semiconductor, planar heterojunctions, bulk heterojunctions and tandems- will be covered. All of the important processes that occur in the cells will be addressed, including: • optical interference and light absorption • exciton diffusion and energy transfer • forward and back electron transfer (exciton splitting and recombination) • charge transport Device modeling and prospects for raising the efficiency from the current level of 5 % to 20 % will be discussed.