The last two and a half decades of exoplanet science have revolutionized our understanding of our place in the Milky Way Galaxy. Starting with the first detection of an exoplanet in 1995, we now know of over 5,000 planets orbiting stars other than the Sun. The observed diversity of exoplanets’ sizes, masses, and orbital architectures has also challenged our conceptions of planet composition, formation, and evolution. The precise characterization of exoplanets, both known and newly discovered, is the first step toward understanding this diversity. To this end, I use photometry from NASA’s Transiting Exoplanet Survey Satellite (TESS) and spectroscopic observations from the 10-meter Keck I telescope on Maunakea to measure the precise radii and masses of exoplanets. In particular, I observe exoplanets that are amenable to future studies of atmospheric characterization with JWST. In an upcoming paper, I will present the masses and radii of 12 newly discovered planets using observations from TESS and Keck. In this paper I also discuss emerging features of the exoplanet mass-radius diagram and how they might relate to the physics of planet formation.