Lecture notes on duality and interpolation spaces

Known or essentially known results about duals of interpolation spaces are presented, taking a point of view sometimes slightly different from the usual one. Particular emphasis is placed on Alberto Calderon's theorem describing the duals of his complex interpolation spaces [A_0,A_1]_\theta. The pace is slow, since these notes are intended for graduate students who have just begun to study interpolation spaces. This second version corrects some small misprints. It also draws attention to a convenient norming subspace of the dual of a complex interpolation space, and to the slight difference between the spaces \mathcal{G}(X_0,X_1) introduced by Calderon and by Stafney.