Plant viruses are of considerable interest to the science of biology, and their study has contributed significantly to the elucidation of several mysteries of traditional and molecular biology. In addition, plant viruses are of great practical significance because they infect plants, cause plant diseases, and result in economic loss. Estimating the economic losses caused by plant viruses has been very difficult. This chapter analyses the types of losses caused by plant viruses, to estimate the costs of these losses, to examine the relationship of the losses to the mode of virus spread in nature, then describes some specific losses caused by viruses on some crops, and to estimate the economic costs of management of plant virus diseases. Virus-infected plants often have fewer fruit and seeds than healthy plants. Since virus infections of plants often cause economic loss, measures are taken to produce plants free of viruses or to protect them from becoming infected with viruses.