The primary goal of the GM/CA@APS Resource is to provide user access to cutting-edge macromolecular crystallography beamlines to determine 3-D atomic structures of forefront problems in structural biology. The Resource was founded in 1999 and has been in operation as a national user facility since 2004, providing a full array of cutting-edge technologies for macromolecular crystallography in Sector 23 at the Advanced Photon Source (APS), Argonne National Laboratory. GM/CA is led by Robert Fischetti (Argonne National Laboratory, contact PI) and Janet Smith (University of Michigan, PI) with an effective administrative structure to operate, maintain, and upgrade the beamlines, and run a high-impact user program. The GM/CA macromolecular crystallography core provides a modern, powerful environment with sophisticated tools for sample screening, interrogation and data collection in fixed-energy, SAD and MAD modes through operation of two insertion-device X-ray beamlines, 23ID-B and 23ID-D, from canted-undulator sources. The beamlines are independently tunable over a wide energy range and are built to deliver a brilliant X-ray beam with exceptionally stable position and intensity. The exceptional stability makes GM/CA an ideal facility for anomalous scattering, and users can quickly (few minutes) change energy at will within the 5.0-20 keV range (λ = 2.48 Å - 0.62 Å). The stable beam enables a powerful microcrystallography capability. Small beams for microcrystallography are created by a quad mini-beam collimator with user-selectable beam sizes of 5-, 10- and 20-micron diameter in addition to the full beam. Users can rapidly (within seconds) change beam size for any sample at any time without beam realignment. The beamlines are equipped with fast, low-noise pixel-array detectors, an Eiger 16M on 23ID-B and a Pilatus3 6M on 23ID-D. Cryo-protected samples are robotically mounted from a 288-sample Dewar and can be visualized and manipulated in the beam with micron precision. Users control the beamlines and samples with the intuitive and biologist-friendly JBluIce graphical user interface. JBluIce includes GUI-based functions for beamline control, sample centering, sample screening, data collection, X-ray fluorescence energy scan, raster scan of the sample, and data analysis. The powerful raster feature is used to scan fields of micro-crystals or to find the best diffracting region of a larger crystal. Information from raster scans can be shared with the data collection routine for a variety of data collection options. The data analysis routines provide unit cell and resolution estimates to formulate a strategy for data collection, which is automatically coupled to three data processing pipelines, providing users with feedback in near realtime. During the 2016-2018 period, a total of 857 unique individuals from 157 unique groups used GM/CA beamlines, and annually interrogated 35,000 samples, deposited 225 structures in the PDB, and published 130 papers (12%...