As cannabis use increases, so does the need for treatment. There is a current lack of evidence-based treatment programs for heavy cannabis use, and challenges around recruitment and retention for extant programs are common. The goals of this dissertation were to develop an online treatment program for adults with heavy cannabis use and examine the efficacy of a therapist-guided introduction. Study 1 (N = 152) was a 3-arm RCT examining the efficacy of CANreduce, a 6-week, self-guided program using principles of cognitive behaviour therapy and motivational interviewing. Participants were randomized into a motivational enhancement therapist guided (MET) introduction, a nontherapist (non-MET) research assistant guided introduction, or waitlist control. Assessment data were collected at baseline, end of treatment (6 weeks) and follow up (10 weeks). All participants reduced their cannabis consumption frequency, quantity, and cannabis-related problems at 6 and 10 weeks. Participants in the MET-therapist condition showed significantly greater reductions in cannabis quantity compared to the control. Participants in the non-MET research assistant condition showed significantly greater reductions in cannabis problems compared to control. There was no significant effect of condition on cannabis frequency, anxiety, depression or quality of life. Study 2 examined the challenges in recruiting and engaging individuals in the CANreduce program. Despite following the core elements of published treatment retention protocols, significant recruitment challenges were experienced. Of the 801 people that completed screeners, 31.3% (n = 251) were eligible for the program. Of those eligible, 54.3% (n = 51) assigned to the MET therapist condition and 45.7% (n = 43) assigned to the non-MET research assistant condition initiated treatment. Treatment initiation predictors included higher cannabis use problems score, lower family history density, increased alcohol use frequency, and more positive attitudes towards treatment. Treatment engagement (i.e., percentage of program completed) predictors included increased social motives for cannabis use and a more positive attitude towards treatment. Overall, this dissertation provided initial evidence for the Canadian CANreduce program, benefits of the MET therapist guided introduction, as well as provided insight into the difficulties recruiting and engaging individuals with heavy cannabis use in online treatment.