Mobile Data 101: Challenges and Best Practices` is a guide for developers, designers, and data scientists working with mobile data, particularly GPS and Telco data. The article highlights the challenges of using these two types of mobile data, including accuracy vs precision issues and biases in the data. It also discusses privacy and security concerns when working with mobile data, emphasizing the importance of anonymization and aggregation techniques to ensure responsible use. Despite its challenges, mobile data offers a powerful tool for decision-makers, providing unprecedented insight into human movement and behavior patterns.