In this article we discuss the state of the art in the field of second-order non-linear optical polymers. More specifically, we highlight those results that we think made an important contribution to the field, combined with some of our own results. We start with a general overview of all the aspects involved in characterizing second-order nonlinear optical polymers, from thin film formation and poling to second-harmonic generation and electro-optic measurements on such systems. Next, we review the second-order non-linear optical properties of selected polymer systems such as poly(vinyl ether)s, polystyrenes, polymethacrylates, main-chain polymers and high T-g polymers like polyimides and polymaleimides. Finally we discuss some new polymer systems that might become in the field of non-linear optics in the near important future.