The Model and ObservatioN Evaluation Toolkit Input Output (MONETIO), developed at NOAA Air Resources Laboratory is an open source python package to process a variety of chemical transport models and air composition measurements. MONETIO was originally part of the Model and ObservatioN Evaluation Tool (MONET) but was later moved into its own codebase to provide a more flexible and agile development path. MONETIO provides a flexible python package to read, process and format observational data from point measurements like the NASA AErosol RObotic NETwork (AERONET) or EPA AirNow to flight data in ICARTT format. MONETIO is also able to read gridded datasets from an array of chemical transport models such as WRF-Chem, MUSICA, CMAQ and the UFS. MONETIO is well-modularized and can add further observational datasets and different models. The tool is accessible to everyone including university and national laboratory researchers, as well as graduate students and postdocs. The goal is to provide the tools to evaluate research, operational, and regulatory models against a variety of observations including surface, aircraft, and satellite data all within a common framework.