Polymer/polymer blend solar cells based on a blend of two types of conjugated polymers acting as an electron donor (hole transport) and acceptor (electron transport) have recently attracted considerable attention, because they have numerous potential advantages over conventional polymer/fullerene blend solar cells. The highest power conversion efficiency (PCE) was slightly above 2% five years ago, whereas PCEs of beyond 8% are the state-of-the-art today, and the efficiency gap between polymer/polymer and polymer/fullerene systems has closed very rapidly. In this review, we provide an overview of recent progress towards the performance enhancement of polymer/polymer blend solar cells. In addition, we discuss the future outlook and challenges regarding PCEs beyond 10%.