Electronic nicotine delivery systems (ENDS) are used by millions of Americans, however, their long-term health effects are unknown. Although ENDS use delivers far lower levels of carcinogens than tobacco smoking, very little is known about whether ENDS use affects the risks of developing cardiovascular (CV) or pulmonary diseases, two of the most highly prevalent causes of morbidity and mortality in the United States. This research proposal will quantify the effects of long-term ENDS use on validated and novel biomarkers of CV and pulmonary disease and how they are influenced by use heaviness, age, body weight, and co-use of other products. This research also will shed additional light on long-term ENDS use patterns and ENDS dependence. The importance of this study does not rest upon demonstrating adverse effects of ENDS use. The study team will recruit 400 long-term, stable users of ENDS and 200 age- and gender-matched control participants who do not use ENDS or combustible cigarettes. They will undergo comprehensive biomarker assessments over 36 months. Biomarkers assess CV and pulmonary health status and risk and include vital signs, fasting blood samples for systemic inflammation and oxidative stress, lipids, and markers of insulin resistance and cardiometabolic health. Arterial structure changes will be assessed using carotid ultrasound and brachial artery flow-mediated dilation. Sympathetic nervous system activation will be assessed by heart rate variability and arterial diameters. For pulmonary measures, the study team will obtain non-contrast quantitative computed tomography (CT) images to assess air trapping and texture-based measures of inflammation. The researcher also will perform spirometry and measure exhaled nitric oxide. Integrated cardiopulmonary health will be assessed via treadmill stress testing. Other measures will include exhaled carbon monoxide, real-time measures of nicotine product use, nicotine dependence, and use of cannabis and alcohol. Biomarker status over time will be compared across groups and related to ENDS use heaviness, use of other products, and person factors (i.e., age, gender, race/ethnicity, weight, and smoking history). About 130 participants who were phenotyped from 2019-2021 will be included. The Primary Aim is to determine relationships between ENDS use heaviness and changes in our CV and pulmonary biomarkers over 3 years. Biomarker status across ENDS users and non-users over time will be compared and associations between biomarker status and ENDS use heaviness within the ENDS user group will be examined. The Secondary Aim is to characterize changes in ENDS use patterns and dependence over time and to determine how these are related to biomarker status changes and how they are influenced by the person factors described above. The primary and secondary CV measures are changes in carotid intima-media thickness and grayscale median. The primary and secondary pulmonary measures are the changes in the quantitative CT measures of air trapping and parenchymal texture. This proposal will produce the most informative evidence to date on how long-term ENDS use affects CV and pulmonary health and disease risk.