We discuss the small-b_T expansion of transverse momentum dependent parton
distribution functions (TMD PDFs). The consideration is based on the operator
product expansion of the non-local operators over the transversely local
operators. Our approach allows us to obtain explicit dependance on both
(transverse and collinear) factorization scales, as well as evolution equation
for the higher b_T corrections. For the presentation of the technical
difficulties and their solutions we consider the toy-model $\phi^3_{D=6}$.
We also show the main results for TMD PDF in QCD, and discuss advantages of
our approach.