Vygotsky's concept of ZPD (Zone of Proximal Development) has been recently applied to the context of language teacher education by a number of researchers (e.g. Ohta, 2005; Singh & Richards, 2006; Nassaji & Cumming, 2000). Besides Vygotsky's notion of ZPD, this paper relies on two associated theories from outside the TESOL discipline namely; Valsiner's (1997) Zone theory and Blanton, Westbrook and Carter’s (2008)illusionary zone (IZ)to  provide a broad picture of the most influential variables facilitating or constraining teacher's professional development or, as specifically defined here, his ZPD progression. Several excerpts in the form of teacher comments taken from different qualitative studies in the related literature are given to lay evidence for the effect, if any, of the raised variables. Finally, the paper concludes with some practical tips for the pre/in-service language teachers as to how to keep their evolving ZPD dynamic in their long-life language teaching profession.