Abstract Subwavelength lattices of metallic patches and strips represent one of the simplest architectures for realizing capacitive and inductive metasurfaces, respectively. In spite of their simplicity, when placed on top of a grounded dielectric slab to form a Gires–Tournois étalon, a variety of physical phenomena can be observed: “ leaky ” and “ plasmonic ” resonances, “ bound states in the continuum ”, frequency‐shifting of “ Fabry–Perot ” resonances, just to name but a few. In this work, all these phenomena are systematically investigated under a common and rigorous electromagnetic framework, clearly demonstrating their subtle connection and physical meaning. Several metasurfaces, showing either a capacitive or inductive behavior, are fabricated and their reflective properties are experimentally validated in the terahertz (THz) spectrum, through THz time‐domain spectroscopy. Remarkably, an original measurement technique is proposed to determine the sheet impedance of the metasurfaces by exploiting the frequency‐shifting of the leaky resonances. In this respect, a sensitivity figure of merit is also defined to quantitatively assess the accuracy of the proposed technique. The results of this study are expected to serve as a reference for the design of metasurface‐based devices such as absorbers, antennas, filters, and sensors, which play a key role in emerging, interdisciplinary THz applications spanning from security screening to next‐generation wireless communications.