A non-randomized, prospective study of up to 150 subjects with asthma. Patients will perform eNO testing with the Apieron INSIGHT(tm) eNO system and the NIOX eNO system at the physician's office/clinic after a short demonstration and training by a healthcare professional trained in the use of the devices. Exhaled NO testing involves inhaling to lung capacity, and then performing a controlled exhalation into a mouthpiece connected to each of the Apieron INSIGHT(tm) and NIOX systems. Patients will perform an eNO measurement with the Apieron INSIGHT(tm) system and with the FDA cleared NIOX system for the accuracy portion of the study. Patients will perform 2 sequential eNO measurements on each system for the precision part of the study. Subjects that measure eNO values over 100 ppb will be asked to repeat 2 sequential eNO measurements on each system.