As defined by the Environmental Restoration Program, an assigned piece of work to be finished within a certain time. A task is a unique entity of work that addresses either the assessment effort or cleanup effort for remediating an OU. An assessment task can consist of up to four subtasks representing the first four remediation phases, usually accomplished in series. A cleanup task consists of two subtasks representing the remaining two remediation phases. The subtasks are the basic building blocks in the program and as such, are individually planned, approved, executed, and controlled. It is defined at the fourth WBS level: remediation task summary.