The details of the size and shape of nanoparticle (NP) catalysts significantly impact their catalytic activity and effectiveness. Thus, the ability to characterize these materials at the relevant scales is critical to the rational design of improved catalysts. High-angle annular dark-field scanning TEM (HAADF-STEM) is particularly suited for the study of heterogeneous NP catalysts as it provides directly interpretable contrast primarily dependent upon the atom type and material thickness. Important information about the 3D structure, however, can be lost due to the 2D projection nature of TEM. Quantitative STEM (QSTEM) can recover atomic-scale 3D structural information from a single HAADF-STEM micrograph by taking advantage of the fact that with digital detectors, we are essentially counting electrons. Through careful calibration and measurement of the image and microscope, the contrast (scattered electron intensity) can be explicitly related back to the number of atoms involved in the scattering. This presentation discusses our developments on two different QSTEM approaches, one based on a conventional TEM/STEM and another on an aberration-corrected dedicated STEM.The earliest QSTEM work performed demonstrated the feasibility of counting the number of atoms in ultra-small NP using sufficiently high collection angles (≥100 mrad) [1,2]. It was further shown that this method could also indirectly recover details of NP shape (e.g., spherical, hemispherical, or plate-like) [3]. Microscopes have advanced greatly in the intervening years, however, and the old VG STEMs used in these studies have all but disappeared. Here we present our adaptation of this method to enable its use on a conventional, non-aberration-corrected S/TEM, a JEOL JEM 2100F S/TEM with no special attachments or modifications to the microscope required. Two Au NP specimens, synthesized via UHV e-beam evaporation, were examined in this study: Au NP supported on an ultra-thin carbon (UC) film, and Au NP deposited onto a γ-Al 2 O 3 scaffold. The NPs were <2 nm in size, typically ~1 nm. The necessary calibrations for QSTEM were accomplished by utilizing the free-lens control functionality of the JEM 2100F, allowing for the proper weighting and quantification of the intensity scattered from each NP. From these values, the scattering cross-section for each NP was calculated and the number of atoms determined, c.f., Figure 1. More recently, the introduction of aberration-correctors enabled the development of atomic-resolution QSTEM, wherein by normalizing the intensities of an atomically resolved HAADF-STEM micrograph into units of fractional incident beam current, the number of atoms in each atomic column can be calculated through comparison with image simulations [4]. In combination with energy minimization computations, estimations of the particle's 3D morphology and atomic coordination can be reconstructed [5]. In this presentation we will discuss our development of a QSTEM technique on the aberration-corrected Hitachi HD2700-C STEM at Brookhaven Natio...