In this paper, the mobility of software processes is proposed as a novel concept. It is defined as the structural change in a software process resulting from interactions among linked process elements. The concept addresses the essential change in a software process which brings a high variability and unpredictability to process performance. Three categories of the mobility that lead to the structural change are identified and expounded upon. A reference model for describing the concept is put forward based on the polyadic π-calculus. With the mobility of software processes, it is possible to design a new PCSEE and associated PML with increased flexibilities.