In the past few years, the photovoltaic community has witnessed a fast growth of inorganic-organic hybrid perovskite solar cells (PSCs) due to their exceptional power conversion efficiencies with a certified record value exceeding 25%, which rivals the established commercial solar cell technologies. In the stateof-the-art PSCs, a hole-transporting layer (HTL) typically consisting of an active hole-transporting material (HTM) and some additives or dopants, takes the responsibility of transporting the photo-generated holes from the perovskite to the selective contact electrode, and is a key component for achieving high-performance and stable PSCs. In this review article, recent advances in the development of transition metal complexes in HTLs for PSCs including HTMs and p-type dopants are presented. (C) 2019 Elsevier B.V. All rights reserved.