The photovoltaic industry has grown fast the recent years, but the price per watt has not jet reached grid parity. A further increase in solar cell efficiency is therefore needed. It is suggested in this work that the main quality problem in multicrystalline silicon wafers is the existence of dislocation clusters covering large wafer areas. Dislocation formation mechanisms such as punch-out from precipitates with a thermal expansion coefficient different from that of the silicon matrix, intergranular - and intragranular hardening and development of strain fields due to differences in the elasticity module between different grains are reviewed. For dislocations nucleated by an angular grain boundary a multiplication and growth mechanism is proposed where dislocations can cross slip and line up at certain crystallographic directions during crystal growth. This work would be of interest for anybody wanting to have an introduction to dislocation theory relevant for solar cell silicon, or anybody being interested in improving solar cell efficiency.