Project constraints are factors which limit the project. The three that are most often mentioned are scope, schedule (time), and budget. Other common constraints are quality, resources, and risks.

In many cases, one constraint can be adjusted by changing another. For instance, with an increase in budget, more resources can be added. Many project management tasks involve the monitoring, adjusting, and reporting project constraints.

Scope and quality are also categories of requirements. (See Juran on "Fit for Purpose".) Generally, scope refers to the functional requirements, and quality refers to the usability requirements.