AbstractCrystalline silicon is currently the basis of about 95% of all solar cells commercially available. Multicrystalline silicon makes up over the half of this amount. The complex defect occurrence and interaction of defects in this material offers a wide field of research and a high potential for material improvement. Extended lattice defects such as dislocations and grain boundaries are important as recombination and storage centres for metallic impurities. Their control is essential to obtain high efficiencies of the solar cell. Important parameters for the assessment of the final efficiency of the solar cells are the distribution and structure of the defects and their impact on the lifetime of minority charge carriers. The current understanding of the nucleation mechanisms of the most important defects during crystal growth will be described. Interaction processes between mobile impurities and extended defects are important for the recombination activity. Measurements of the recombination behaviour and the contamination level will be presented. Effects on the solar cell efficiency shall be discussed. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)