process under mild conditions. The abovementioned advantages make perovskites a promising family of materials for the next-generation photovoltaic solar cells. Since the first report on the perovskite solar cell (PSC) by Miyasaka and coworkers in 2009, [1a] considerable efforts have been made to improve the power conversion efficiency (PCE) of the PSC to rival those of commercially available silicon solar panels. [1b-e] In a strikingly short period of time, the highest certified PCE has reached a value as high as 25.2% in a single-junction PSC. [2] Self-assembled monolayers (SAMs) are 2D nanomaterials with the thickness of one or few molecules. [3] Self-assembly of molecules on the surface is a thermodynamically favorable process where molecules interact with each other to form organized structures. Typically, molecules are vertically aligned on the surface with some tilt angle relative to the surface normal. The molecules are designed to have three parts: the anchoring, the spacer, and the terminal groups (Figure 1). 1) Anchoring group: the anchoring group is responsible for the interaction between the molecule and the surface. Various anchoring groups that bind to specific substrates are available, which provides users the option to select the type of electrode and molecule to suit their intended purpose. The most widely studied class of SAMs is derived from the anchoring chemistries between thiol and coinage metals or between silane and oxide substrates. In SAM-inserted PSCs, various oxide substrates are commonly used for bottom contacts, and few Brønsted-Lowry acids (e.g., carboxylic acid, phosphonic acid, and boronic acid) are extensively utilized (see below for details). Anchoring chemistry matters for the tilt angle of molecule with respect to the surface normal, work function (WF) of substrate, interfacial dipole, contact resistance, and energy offset between the Fermi level and energy of frontier molecular orbital. All of them are essential for the electronic function and performance of PSCs. These effects of SAMs on interfacial properties of PSCs are discussed below. 2) Spacer group: the spacer group is the backbone of the molecule, and it bridges terminal and anchoring groups. The length of the backbone is important for electronically isolating one contact from another. The spacer group is responsible for lateral interaction between molecules during the selfassembly process, which affects the final packing structure. Self-assembled monolayers (SAMs), owing to their unique and versatile abilities to manipulate chemical and physical interfacial properties, have emerged as powerful nanomaterials for improving the performance of perovskite solar cells (PSCs). Indeed, in the last six years, a collection of studies has shown that the application of SAMs to PSCs boosts the performance of devices compared to the pristine PSCs. This review describes recent studies that demonstrate the direct advantages of SAM-based interfacial engineering to power conversion efficiency (PCE) of PSCs. This review includes 1) a b...