Conjugated polymers have been regarded as promising candidates in organic photovoltaics owing to the outstanding electronic, optical and mechanical flexible properties. In this chapter, first, we summarize and discuss the requirements and design strategies of donor polymers in polymer solar cells (PSCs). Second, the design concepts and representative donor–acceptor (D–A) copolymer donors based on six types of donor building blocks are discussed. Notably, the blooming developments of novel two-dimensional conjugated polymer donors are emphasized. Third, the design considerations and examples of novel terpolymer donors are introduced. Finally, we summarize the future directions and approaches to developing higher performance donor polymers for photovoltaic applications.