An overview is given on positron annihilation studies of vacancy?type defects in Cd? and Zn?related II?VI compound semiconductors. The most noticeable results among the positron investigations have been obtained by the study of the indium? or chlorine?related A centers in as?grown cadmium telluride and by the study of the defect chemistry of the mercury vacancy in Hg_1-xCd_xTe after post?growth annealing. The experiments on defect generation and annihilation after low?temperature electron irradiation of II?VI compounds are also reviewed. The characteristic positron lifetimes are given for cation and anion vacancies.