Vehicle probe data includes information from mobile cellular devices, fleet vehicles with embedded global positioning systems (GPS) devices, connected vehicle telematics, and mobile applications (cell phone, tablet, or other devices) that track user locations. These data, typically provided by third-party vendors who capture, manage, analyze, and anonymize the data, provide cost-effective information across the transportation system. As such, transportation agencies are increasing adopting the use of vehicle probe data to support the planning and operation of transportation systems. This document serves as a primer on vehicle probe data, including the sources and applications for probe data, the challenges agencies need to address to effectively use the data, and the current and future opportunities for using vehicle probe data.