Data loggers are tiny computers which record temperature, humidity, water flow rates, and other environmental parameters over a long timeframe. How can they be used to diagnose building issues? What are some tricks and tips for using data logging systems? The possibilities of these inexpensive and useful devices will be explored through case studies on condensation issues, air and hydronic HVAC, and building ventilation. We will show techniques for manipulating the acquired data into useful formats, and share lessons learned in real-world investigations.