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Background: Obsessive-compulsive disorder (OCD) is a prevalent and disabling condition. Although effective treatments such as cognitive-behavior therapy (CBT) exist, accessibility to this treatment is low and many OCD patients do not respond to CBT. Aims: The general aim of this thesis was to develop and test new treatment strategies for enhancing both the accessibility and the efficacy of CBT in the treatment of OCD More specifically, the aims were to investigate: • The efficacy of internet-based CBT (ICBT) for OCD (Studies I, II, and IV). • The impact of cognitive interventions in ICBT on obsessive beliefs and OCD symptoms (Study III). • The efficacy of adding an internet-based booster program to ICBT (Study IV). • The efficacy of adding the medication D-Cycloserine (DCS) to ICBT (Study V). Methods: To test the efficacy of ICBT, an open pilot study of a newly developed treatment program was conducted (Study I), followed by a randomized controlled trial (RCT; Study II). Long-term follow-up was assessed at 4-, 7-, 12- and 24-months after ICBT (Study IV). To investigate the role of obsessive beliefs, a longitudinal mediation analysis was conducted on patients receiving ICBT with weekly measurements (Study III). Booster efficacy was tested with an RCT design, where half of the patients who had undergone ICBT were randomized to a three'week booster program (Study IV). To test the efficacy of DCS, a double-blinded RCT was conducted, where 128 OCD patients were allocated to either DCS or placebo as adjuncts to ICBT (Study V). Results: Large effect sizes favoring ICBT were observed (Studies I and II) and these treatment effects were sustained up to two years after completed treatment (Study IV). The cognitive intervention, unexpectedly, immediately increased obsessive beliefs and this, in turn, predicted better treatment outcome (Study III). The addition of an internet-based booster program reduced relapse (Study IV). Although there were some indications of faster treatment response in the DCS group, no beneficial effects of DCS as adjunct to ICBT were observed either at post-treatment or at follow-up (Study V). Conclusions: ICBT is an effective method of administering CBT for OCD. Cognitive interventions in ICBT may have an impact on obsessive beliefs, and play a role in later symptom reduction. There may be beneficial effects of adding a booster to ICBT, but there is no evidence of a beneficial effect through adding DCS to this treatment.