Weeds are a major damaging factor in agriculture. The direct damage that they have on crops is related to their strong competitiveness to the vegetation factors. Their strong adaptability to the environmental conditions makes them a serious problem for the normal growth and development of the agricultural crops worldwide. The harmful influence of the weeds, however, is not limited to their competition with the main vegetation factors. The damage caused indirectly by weeds is mainly that they support the spread of many pests and diseases of crops. This review summarizes data from studies at home and abroad on weed host of economically important diseases and pests on agricultural crops.