critical temperature superconductivity, ferroelectric semiconductor-based cells for solar-energy conversion and others. Analogous to the oxide counterparts, metal lead halide perovskites (MLHPs) (represented by CsPbX 3 , X = I, Br, Cl) also exhibit a cubic lattice in 3Ds at room temperature, as illustrated in Figure 1, but with high-defect tolerance. [9,10] Commonly, their defect tolerance is a major enabling factor for their remarkable optoelectronic properties, such as considerable optical absorption coefficients, long carrier diffusion lengths, and high photoluminescence (PL) efficiency. [11][12][13][14] Furthermore, MLHPs can be synthesized cost effectively on large scales through simple solutionbased processes. As a result, the MLHPs have been investigated for optoelectronic applications in photovoltaics, light-emitting diodes, laser devices, and photodetectors. Recently, the lead halide hybrid organic-inorganic perovskites (HOIPs) have been emerging as a new territory of photovoltaic materials. The HOIPs comprise earth-abundant elements, and yet, exhibit outstanding semiconducting and light-absorption properties. [15,16] In particular, the power-conversion efficiency of HOIP-based devices has surpassed 20% over several years of research and development. [17,18] The HOIPs have a large structural diversity. When the A-site is only occupied by small organic cations (typically methylammonium, MA), they ideally exhibit 3D structures in an octahedral coordination.More interestingly, the HOIPs can exist in layered structures, or 2Ds. 2D HOIPs with unique layered structures are constructed from alternately stacked molecular sheets of organic cations and inorganic anions, as illustrated in Figure 1. The stacking sheets in 2D HOIPs can be regarded as a multiquantum-well structure in semiconductor physics, greatly enhancing the quantum and/or dielectric confinements. As a result, the free charge carriers (electrons or holes) can be strongly confined in atomic-thickness layers, readily forming excitons, which can be treated theoretically as hydrogen-like quasi-particles but moving in 2Ds. As a result, NLO effects can be greatly enhanced along with strong exciton-photon coupling. [19][20][21][22] 2D perovskites have opened up a new opportunity for both fundamental research and practical applications in ultrathin and flexible optoelectronics devices.Moreover, perovskites can also be synthesized into various lower-dimensional structures such as nanocrystals with controllable shapes and sizes. [11,23,24] This allows ones to modulate the quantum confinement effect in the lower-dimensional structures to achieve desirable light-to-electrical conversion, light-emitting, lasing, photodetecting, and NLO properties for various optoelectronic applications.Recently, there have been intensive and extensive research works on perovskites because of their excellent electrical and optical properties. This review is focused on their multiphoton absorption (MPA) and applications. All the MPA coefficients or cross-sections reported in the ...