Publisher Summary The primary instrument for studying heat penetration in retorted foods is the temperature data logger. Data logger systems for use in food processing are available in two primary forms: wired and wireless. In a wired system, temperature sensors, usually thermocouples, are hard wired directly to a recording device that has the required electronics to make the temperature measurement and record the data. In a wireless system, self-contained, battery-operated measurement and recording devices are used that have the required sensor, electronics, memory, and microprocessor to record process data. This chapter illustrates the configuration of a wired and wireless data logger system and addresses the wireless data loggers to study heat penetration in retorted foods. For a wireless data logger, the self-contained loggers are programmed to collect data at a specified rate and time, deployed into the process, and retrieved following the process and then the data is read into a PC. For most retort applications, a wireless data logger system is the instrument of choice because of the ease of setting up the system. Significant advances in wireless data logger technology have been made in recent years, primarily impacting on their size, accuracy, and reliability. Wireless data loggers can also be mounted internally in the container for situations where external mounting would interfere with retort operation.