Device evaluation, which is an essential skill set for the respiratory therapist, ranges from comparing manufacturer's specifications to comprehensive device testing, either with patients or on the bench. Good device evaluations help guide decisions about device selection, procedure development, and risk and failure analysis. Poor evaluations cost time and money and fail to return value. Manufacturer's specifications alone are poor criteria for device selection, because of how and why those specifications are created and the potential gap between the manufacturer's test methods and the complexity of clinical situations. Proper clinical evaluation of devices with patients requires extensive preparation and resource expenditure, and clinical evaluations may not allow isolating key variables to determine specifics of device performance. In vitro testing, using models to simulate discrete components of device/patient interface, is less expensive and easier to conduct. This article discusses the process of experiment design and model development for device and equipment evaluations.