A combination of improved understanding of molecular design criteria and polymer purification techniques, as well as optimised fabrication techniques and device surface treatments, have driven recent advances in the performance of semiconducting polymers for transistor applications. This development has allowed polymer-based devices to reach parity with the best values obtained for small molecule evaporated devices. Herein, we present the most recent work on solution processable high mobility donor-acceptor type polymers. We discuss the approaches that have been taken to improve both hole (and electron) mobility further. We will not only focus on chemical design criteria, but also describe certain processing approaches which have led to impressive hole mobilities of up to 5.5 cm(2) V-1 s(-1).