These are the lecture notes of a seminar held at the Universitat Aut\`onoma
de Barcelona where the Jones-Wolff theorem about the dimension of harmonic
measure in the plane is explained in full detail, for non-expert readers.Comment: The definition of Green's function used in the proof of Lemma 1 in
the previous version of the preprint was only valid for bounded domains. This
is corrected in this new versio