The incessant evolution of organic and hybrid solar cells has demonstrated that a multilayer architecture is required for an optimum device functioning. Although more attention has been traditionally focused on the active layer materials, it is the interfacial materials, placed between the electrodes and the active region, which might play some of the most critical roles in the solar cell. In this regard, interfacial layers can modulate the compatibility between the electrodes and the active layers in terms of morphology, surface energy or energy level alignment. Furthermore, interfacial layers modulate the light absorption working as optical spacers, assist in the exciton confinement and preserve the active layer from damage or degrada tion. However, the most important role of interfacial layers concerns the charge transport, defining an energy gradient for the selective migration of free charge carriers from the active layer to the electrodes. This chapter offers a comprehensive description of those materials specifically working as hole transporting layers in organic, dye-sensitized and perovskite solar cells. Conjugated polymers, small molecules, metals, metal oxides, self-assembled monolayers, carbon nanotubes and graphene-based materials will be discussed along with their influence on different aspects aimed at the optimization of the solar cell performance.