This chapter aims to examine the practices and potential of Activity Theory (AT) for educational technology research (ETR). AT provides a framework within which to understand object-oriented, collective, and social environments (Engeström, Perspectives in activity theory (pp. 19-38). Cambridge: Cambridge University Press, 1999). Activity systems provide flexible frameworks that can be modified according to the nature of the context. In ETR, AT has been used as a tool to analyze and design complex learning situations as well as to analyze the contradictions and barriers in technology integration and to describe the dynamics of organizational knowledge creation. In this chapter, the basics of AT are presented and the available research using AT as a methodological tool in ETR is examined. The use of AT as a metaphorical tool in learning design and artifact development, as an analytic tool in an innovation study, and as a descriptive and prescriptive tool in a knowledge management study is explained. I also refer to the potential use of Actor Network Theory (ANT) as a possible third generation AT (Engeström, Journal of Education and Work 14(1):134-156, 2001) that can be used to understand the symmetrical relationship between multiple activity systems.