Plant virus replication and spread require specific interactions between the obligate organism, the virus, and its host, the plant. Both virus and plant factors, whether they are involved in virus accumulation or in defense against virus accumulation, require specific environmental conditions to maximize their activity. In order to understand how virus and plant factors function, it is therefore necessary to understand and control the environment that influences their activity. Temperature, light, moisture, air movement, soil status, and plant nutrition are the major environmental factors that influence a virus' accumulation in its host. This unit discusses each of these environmental factors, documenting their influence on virus accumulation in the plant, and discusses methods to best control and monitor their variation. Controlling the environment will limit experimental variation and lead to more meaningful and reproducible experimental results.