ATLAS is a free, publicly available, web based, open source software tool for researchers to conduct scientific analyses on standardized observational data converted to the OMOP Common Data Model V5. Researchers can create cohorts by defining groups of people based on an exposure to a drug or diagnosis of a particular condition using healthcare claims data. ATLAS has vocabulary searching of medical concepts to identify people with specific conditions, drug exposures etc. Patient profiles can be viewed within a specific cohort allowing visualization of a particular subject's health care records. Population effect level estimation analyses allows for comparison of two different cohorts and leverages R packages.