Recent results of positron annihilation studies of organic solids are reviewed. The studies concentrated around two problems: the models of pick-off process and influence of external factors on positronium lifetime (or intensity) in solids. Several variants of the pick-off model appeared and the geometries other than the spherical geometry were discussed. The positron annihilation method has proved its usefulness in investigations of structural changes in the medium. However, despite of 20 years which have passed since the first attempts to obtain positronium trap properties from annihilation experiments, the situation is not fully clear; new effects are observed at extreme temperatures and under pressure.