In this position paper, we introduce the situated activity model, an activity theory informed activity-based computing (ABC) model that unites the description of activity and situated contexts into one computational model. By introducing a unified activity model, we seek to connect cross domain activity systems ranging from desktop systems to pervasive computing and beyond. In this paper, we describe (i) the situated activity model (SAM), (ii) a conceptual description of a generic cloud-based architecture for the prototyping and development of situated activity systems (SAS) and (iii) the value of the situated activity approach in different application domains.