During melt-growth of macroscopically dislocation free bulk silicon crystals (floating-zone and Czochralski technique) microdefects can form due to the condensation of thermal point defects (self-interstitials, vacancies). The formation of these imperfections, generally referred to as “swirl defects”, is strongly affected by the growth conditions (e.g. the crystal pulling rate) and crystal purity. The various reported defect formation models will be discussed. Special attention will be paid to the effect of doping on swirl defect formation.