This chapter focuses on materials development and highlights fundamental considerations and significant advances in the design of new p-conjugated materials with a specific focus on p-conjugated polymers and various blends thereof. It discusses underlying design strategies toward efficient charge transport for both n- and p-type materials covering, for example, backbone modifications including extended polycyclic aromatic motifs and donor-acceptor structures, side chain modifications, substitution of sulfur with heavier chalcogens, and incorporation of non-covalent conformational locks. The chapter analyses other aspects that play an important role for charge transport in polymeric systems including the role of polymer molecular weight, impurities, additives, and device processing conditions. It highlights examples where significant non-idealities have been reported for certain structures. Nitrogen-containing heterocycles such as pyridine and thiazole provide another means to introduce electron-deficient moieties. This is exemplified with PDBTz, which comprises the thiophene-flanked diketopyrrolopyrrole with a bithiazole comonomer.