Currently recruiting cigarette smokers (any level of smoking) as well as healthy controls for a study comparing breath carbon monoxide (CO) monitors. Breath carbon monoxide can be used as an indicator of recent smoking and is measured by CO monitors. Breath CO can also be used to determine abstinence from smoking. There are several breath CO monitors commercially available, including a next generation model that connects directly to a smartphone and interfaces with a mobile application. Given the importance of comparability between different CO monitors, this study involves a comparison between three monitors to assess their agreement in CO readings and detecting abstinence from smoking. Participants will present for 10 brief study visits to complete CO breath tests using three different monitors. These study visits will last approximately 10-15 minutes.