PID stands for Proportional Integral Derivative. Fancy name, but it’s really just a computer controlled mechanism on an espresso machine that keeps the temperature of an espresso machine stable and allows the user to increase or decrease the set temperature on demand depending on the desired brew temperature or steam temperature. Espresso machines that have PIDs generally cost more than comparable machines that don’t have PIDs. Machines that don’t have PIDs usually have pressurestats, which can be good for keeping temperature stable, depending on the quality, but they aren’t as easily adjustable and don’t have a temperature display in most cases.