In this contribution the use of electroluminescence imaging, bias-dependent lock-in thermography, special dark and illuminated lock-in thermography techniques, and electron microscopy techniques is demonstrated for investigating the physical mechanism of breakdown in multicrystalline silicon solar cells. Two dominant breakdown mechanisms are identified, which are breakdown at recombination-active crystal defects, showing a relatively soft breakdown, and avalanche breakdown at dislocation-induced etch pits, which occurs very steep (hard breakdown) and dominates in our cells at high reverse bias.