The current paper examines the instructional implications of Vygotsky's (1978) seminal notion of Zone of Proximal Development, originally developed to account for the learning potential of children, and investigates ZPD applications to the concept of teacher professional development. Specific attempt has been made to see how a number of assets at the teacher's disposal namely diary writing, peer and mentor collaboration, action research, practicum and TESOL discourse can serve as scaffolders to affect the progression of ZPD in language teachers. The contributions of ZPD to the concepts of scaffolding and dynamic assessment (DA) are explored extensively and the controversial issues are addressed. There is a consensus that the notion of the zone of proximal development and socio-cultural theory of mind based on Vygotsky's ideas are at the heart of the notion of scaffolding. This study highlights the limitations of the metaphor of scaffolding in interpreting the zone of proximal development. The concept of ZPD, as seen through the approach of DA, offers an operational view of the learners' actual level of development and a measure of emerging and imminent development. Utilizing the concept of ZPD, DA unites traditional assessment, instruction, intervention, and remediation. Though the concept of ZPD provides an attractive metaphor for designing instruction and analyzing learning, it poses a real challenge when put into practice. The present research highlights a procedure to provide a more tangible account of ZPD, but research on this area is scanty and further explorations and investigations are needed to reflect the implications of ZPD in instructional context.