This application is being submitted in response to the Notice of Special Interest (NOSI) identified as NOT- CA-22-070, Administrative Supplements for Examining Patterns of Tobacco and Cannabis Use. The prevalence of cannabis use is elevated among adults who smoke, and particularly among those with low socioeconomic status. However, the impact of cannabis use on smoking cessation remains unclear. The proposed administrative supplement will leverage an ongoing smoking cessation intervention trial for low-income adult smokers (R01CA251451) to collect new data to characterize baseline and daily cannabis and tobacco use and co-use patterns, compare cannabis users and non-users on respiratory symptoms and mental/physical health, and evaluate the impact of cannabis use on daily smoking and point prevalence smoking abstinence at post-quit follow-up. Data from 135 adults enrolled in the ongoing parent trial will be included in the proposed analyses, to be completed within the 1-year supplement time frame. All participants will provide information about their tobacco and cannabis use at baseline, prior to initiating smoking cessation treatment, and they will complete smartphone-based assessments of their expired carbon monoxide (CO), self-reported tobacco and cannabis use, and other relevant factors over a 7-week period (1- week pre-quit day through 4 weeks post-quit, and during post-quit weeks 8 and 12). Follow-up assessments of CO- verified 7-day point prevalence smoking abstinence will focus on follow-ups at 4, 8, and 12 weeks post-quit to fit within the 1-year supplement time frame. The supplement will address the following specific aims: 1) Characterize cannabis use characteristics among low-income adults participating in tobacco cessation treatment, including a) baseline and daily prevalence, frequency, intensity, and modalities of cannabis use (e.g., combusted vs. non-combusted) as well as reasons for use, b) baseline cannabis dependence severity and harm perceptions, c) daily prevalence of tobacco and cannabis co-use assessed via smartphone-based daily diaries, and d) interest at baseline and follow-up in reducing or abstaining from cannabis use; 2) At baseline, compare past 30-day cannabis users with non-users on sociodemographic characteristics, tobacco use history, physical and mental health, alcohol use, respiratory symptoms, exposure to cannabis secondhand smoke, expired CO levels, and cigarette dependence; 3) Evaluate the within-day and distal relationships between cannabis use and smoking cessation, including: a) the relation between baseline cannabis use (past 30-day use vs. non-use) and smoking cessation outcomes at follow-up (4, 8, and 12 weeks post-quit), b) the within-day relations of cannabis use with cigarette craving, withdrawal symptom severity, and smoking during the weeks following a scheduled quit attempt via smartphone-based assessments, and c) the relation of cannabis use frequency (i.e., near daily use, days of use) during the...