Experience with wildland fires soon teaches that no two are exactly alike. Fire behavior is not an independent phenomenon-it is the product of the environment in which the fire is burning. Environment has been defined as 'surrounding conditions, influences, and forces that influence or modify.' In applying this definition to fire, we can then regard fire environment as the conditions, influences, and modifying forces that control the fire behavior. Fire behavior must obey physical laws. We consider certain types of fire behavior unusual or unexpected only because we have failed to evaluate properly the conditions, influences, and forces that are in control. To predict fire behavior, and to control and use fire effectively and safely, we must understand and use the interactions of fire with its environment. In the following discussion we will examine the fire environment-what it is, how it varies and why, and how fire itself alters the total picture.