Abstract: Requirements tasks, requirements activities, requirements engineering processes and requirements engineering processes system are formally defined.Requirements tasks are measured with information entropy.Requirements activities, requirements engineering processes and requirements engineering processes system are measured with joint entropy.From point of view of requirements engineering processes, microcosmic evolution of iteration and feedback of the requirements engineering processes are modeled with condition-event nets.From point of view of system engineering, macro evolution of the whole software requirements engineering processes system is modeled with dissipative structure theory.