This research program (EmetStudy) extends emetophobia-related knowledge in seven key areas and achieves three milestones in the treatment of emetophobia. The first milestone is that it is the largest emetophobia treatment study conducted to date. There was a total of 107 participants completed treatment using an online cognitive behavioural therapy (CBT) program, administered by a single therapist over a 12 month period. Second, two new measures of emetophobia related symptoms were developed and validated using a large pool (N = 459) of emetophobia treatment seekers. Finally, EmetStudy led to the publication of the first, and currently the only non-retrospective, structured clinical interviews (N = 64) to ascertain emetophobia comorbidity rates, confirming that comorbidity rates may be lower than previously suggested. Overall this thesis examined seven key research areas that included two treatment goals and five theory related goals, outlined below. The two treatment goals were: (1) to assess the effectiveness of an online cognitive-behavioural internet therapy developed specifically for the treatment of emetophobia and (2) understand the progress of participants during treatment by quantifying commencement, dropout and completion of treatment. The treatment program was conducted in two phases with a Pilot and Main Study.