Abstract: The ExoMars Trace Gas Orbiter was launched in 2016 to make a detailed study the atmosphere of Mars with the key objective of searching for as-yet undetected trace gases diagnostic of active biological or geological processes. It successfully reached Martian orbit in the fall of 2016 and began its nominal science phase in April 2018. By spring next year, it will have made three full Martian-years of observations. In this presentation, I will give an overview of the ExoMars mission and present the key findings, especially my own work, from the Atmospheric Chemistry Suite (ACS) solar-occultation instrument. ACS has a near-infrared channel that makes frequent measurements of the vertical structure of water vapour and temperature, and has presented a look at the contemporary climate and water cycle on Mars in unprecedented details. The mid-infrared channel is the most sensitive instrument to investigate Martian atmospheric composition to date, and with it we were able to observe the vertical structure of carbon monoxide (CO) for the first time. A key target of the ExoMars mission was to solve the mystery of methane (CH4) on Mars, and to-date this spectral signature eludes us. In its place, we have detected a previously-unknown spectral band of carbon dioxide (CO2) and much stronger ozone (O3) absorption features than expected. Other organic molecules, or those bearing nitrogen or sulphur have also not yet been definitely detected. However, we have discovered hydrogen chloride (HCl) in the atmosphere, a target gas expected to be a sign of active volcanism or magmatic outgassing. HCl exhibits a seasonal cycle and we are working hard to better understand its governing photochemistry.