Worry can be thought of as the cognitive component of anxiety. We tend to worry when we aren't sure what's going to happen but we think that we may experience a negative event, such as a failure, loss, illness, or injury. Worry represents an attempt to engage in mental problem-solving on an issue whose outcome is uncertain, but contains the possibility of one or more negative outcomes; consequently worry relates closely to the fear process.

Human beings have the amazing ability to mentally simulate future events: ‘thinking ahead’ means that we can anticipate obstacles or problems, and affords us the opportunity to plan effective compensatory actions. To the extent that it helps us to achieve our goals, ‘thinking ahead’ can be adaptive. Worrying is one form of thinking about the future. It has been defined as thinking about future events in a way that leaves you feeling anxious or apprehensive. Clinically, excessive worry is the primary symptom of generalized anxiety disorder (GAD). Evidence seems to indicate that the content of worry in people with GAD concerns similar topics to ‘everyday’ worry, but that it is often concerned with more unlikely or remote events. The terms ‘hypothetical worry’ and ‘real event worry’ are often used to describe this distinction.  Other important differences are that people with GAD often experience their worries as uncontrollable, and will worry habitually instead of in response to particular triggers.