The extended (~100 kpc-scale) gaseous halos surrounding galaxies, known as the circumgalactic medium (CGM), regulate the gas exchange between galaxies and intergalactic space. Galaxies accrete gas in order to grow and fuel star formation. Energetic events arising from these growth processes, like supernovae (SNe) and supermassive black-hole activity, can expel heavy elements back to the CGM and beyond, regulating future accretion and the course of galaxy evolution itself. While great progress has been made in understanding how galaxies grow, a self-consistent explanation for the origin and fate of the CGM remains elusive. My work leverages high-quality spectroscopic data from large ground-based and space telescopes to obtain the most stringent constraints on the physical conditions of the CGM and address long-standing puzzles on its role in galaxy evolution. In this talk, I will highlight my recent work studying the CGM of a diverse population of galaxies at z<1. I will show that the CGM is multiphase, with large variations in gas abundances and densities that indicate a multitude of physical origins. I will demonstrate that the observed metallicities and elemental abundance ratios of the gas convey critical information about the origin of CGM gas. For instance, while gas in the inner CGM has been significantly influenced by SNe Ia feedback, gas farther out in the CGM (>100 kpc) originates from recent accretion from the intergalactic medium. I will also illustrate that the surprisingly gas-rich CGM of quiescent galaxies, long-thought to be gas-poor systems, has much to teach us about the co-evolution of galaxies and their gas. Finally, I will end with exciting new results from an ongoing study on how galaxy environment influences the gaseous properties of massive galaxies on both small (ISM) and large (CGM) scales.