In a foundational paper, Coifman, Rochberg and Weiss characterize the norm of the commutator [b, T] - where T is a Calderon-Zygmund operator - in terms of the BMO norm of the symbol function b. In this talk, we discuss a two-weight version of this result. Such a result was first obtained by Bloom in 1985, in the one-dimensional case, for the Hilbert transform. More recently, this was extended to the n-dimensional case, for all CZOs, using the modern methods of dyadic harmonic analysis.