A bewildering array of HMEs are available, suited to many different tidal volume and other clinical requirements. Choosing among the different devices depends on many attributes. Grouping different HMEs together to describe their performance may be inappropriate. Independent tests on HMEs provide information to help when choosing among different devices, but care should be exercised to prevent inappropriate comparisons being made between results from studies using different test procedures and protocols. The use of filters in anesthesia is recommended in some countries. Some HMEs have a high moisture-conserving performance, but the use of HMEs may be inappropriate for some patient groups, and other humidifying devices may be more suitable.