Combustible tobacco use remains the primary cause of cancer morbidity and mortality in the United States, and a major cause of cardiovascular and pulmonary diseases. Although there has been tremendous progress in reducing cigarette smoking, approximately 15% of adults over age 18 in the U.S. currently smoke cigarettes, and progress with smoking cessation may be stalling. To reduce the substantial toll from tobacco, we need to consider a variety of approaches to more rapidly reduce its harm, especially for those smokers who may not be motivated or ready to stop smoking or who have been unsuccessful in quitting with current cessation treatments. The primary purpose of this study is to examine whether conventional, combustible cigarette smokers can reduce their use of cigarettes and associated harms by switching to an e-cigarette (Standardized Research E-Cigarette or SREC). This project will use Ecological Momentary Assessments (EMA) to gather real-time, naturalistic reports of conventional smokers' use and experience of SREC as they attempt to switch from combustible cigarettes to SREC. We will examine how the proximal contexts and subjective experiences surrounding SREC use differ from smokers' patterns and responses to conventional cigarette use, and how these patterns of differences may predict success in switching. In addition, we will assess in-depth, in real time, smokers' reports of urges and withdrawal symptoms as they switch to SREC. We will also evaluate changes in health and biomarker variables. We will enroll a sample of 180 adult combustible cigarette smokers who are interested in switching to e-cigarettes. Participants will complete a baseline one-week EMA wave while they are smoking their usual cigarettes, complete baseline assessments, and then will be randomized (within sex and use of menthol cigarettes) in a 2:1 ratio to either a nicotine SREC or a placebo SREC. Participants will receive brief behavioral counseling about switching, and after an initial week of using SREC, they will complete another EMA week assessing patterns and responses to use of both SREC and cigarettes. Participants will receive 12 weeks of SREC; compete daily reports of product use along with bi-weekly check-ins through the end of the 12-week trial, and then complete a one-month follow-up. Our aims are to: 1) Examine whether conventional cigarette smokers can significantly reduce their combustible cigarette use by switching to SREC; 2) Examine whether reductions in conventional cigarette use and uptake of SREC are associated with changes in health and biomarker variables; 3) Examine smokers' subjective responses to SREC in real time, as they are used, and whether these responses are associated with cigarette reduction; and 4) Examine how patterns of use (e.g., contexts, timing) and subjective experience with SREC differ from conventional cigarette use, and whether differences are associated with success in switching to SREC. Data from this study will provide valuable information about the potential for harm reduction if smokers switch to SREC.