ConspectusDuring surface plasmon-mediated light-matter interactions, external energies on plasmonic metal nanostructures undergo energy dissipation via elastic e-e scattering, radiative luminescence, and nonradiative processes such as thermal relaxation (phonon) and electronic excitation (electron-hole pairs). In this process, surface plasmon decays dominantly through nonradiative recombination when the metal is smaller than 25 nm, forming hot carriers, including hot electrons and hot holes, with high kinetic energy of 1-3 eV. Although the ultrafast dynamics of hot carriers are on time scales ranging from femtoseconds to picoseconds, these fast-disappearing hot carriers can be collected as the steady-state photocurrent or chemicurrent by adopting the metal-semiconductor (M-S)-based platform for detecting hot carrier flow. Plasmonic hot carriers, especially as they convert to an electrochemical signal, are a promising topic, and their energy conversion mechanisms are being actively studied in the fields of renewable energy, optoelectronics, and photocatalysis. Recent studies have demonstrated that these hot carriers can both improve the performance of solar energy conversion and control the catalytic activity or selectivity by directly participating in the photoelectrochemical (PEC) reaction.In this Account, we describe the inherent relationship between hot carriers and surface plasmon as well as what role hot carriers play throughout the catalytic reaction. The recent experimental work and the theoretical analysis of in situ hot carrier generation on Au nanostructures were conducted with photoconductive atomic force microscopy and finite-difference time-domain (FDTD) simulations, respectively. We highlight the recent nanoscale visualization of hot carrier flow occurring through light-matter interactions and that the localized surface plasmon field surrounding the Au nanostructure leads to boosted hot carrier generation. In addition, we highlight the recent demonstration that plasmonic hot carriers prolong the lifetime of photoexcited carriers in the MAPbI3/Au/TiO2 hybrid nanodiode by the synergistic effect between plasmonic Au and perovskites. From this work, the solar-to-electron conversion performance of this nanodiode significantly increases due to the amplification of light absorption, which helps to design hybrid platforms for efficient hot carrier photovoltaics. We discuss the application of surface plasmon-driven hot electron generation, including hot electron-based photovoltaic devices and photocatalysts. We highlight the recent photoelectrochemical measurements on the Au/p-GaN heterostructures that are controlled by participating plasmonic hot carriers in the water splitting reaction. Furthermore, controlling the flow of both hot electrons and holes by developing hybrid platform configurations for hot carrier applications has promising opportunities for regulating the catalytic activities of hot carrier-based photocatalysis and improving the photoconversion efficiency of hot carrier-based optoelectronic devices.