Vygotsky introduced the Zone of Proximal Development (ZPD) in the 1930’s. ZPD activity involves semiotic mediation of mind and is a unit of analysis in sociocultural theory (SCT) and activity theory. ZPD activity reveals a learner’s potential future development. Higher cognitive development, including second language acquisition (SLA), involves social processes as learning-leads-development in ZPD activity, where developmentally appropriate support (from other persons or from semiotic tools) enables functioning beyond a learner’s independent skills. Too much support, beyond a learner’s ability to understand, is outside of the ZPD and does not reveal developmental trajectories. Early L2 research on ZPD activity focused on identifying ZPDs using discourse analytic approaches. Promoting ZPD activity is at the core of concept-based language instruction (C-BLI) and dynamic assessment (DA), two interventionist pedagogical approaches. Process-oriented research most effectively reveals ZPD activity. Instead of mentioning ZPD activity, SCT researchers may discuss mediation; ZPDs may also be evident in non-SCT process-oriented studies. Examples of research relevant to ZPD activity include work focused on identifying and understanding ZPD activity, studies on explicit instruction (C-BLI) and instructional assessment (DA), and process-oriented studies on interaction between people and other persons or semiotic tools, including digital tools.