Obsessive compulsive disorder (OCD) affects 1-3% of the population and is the fourth most common mental disorder. It is characterized by a preoccupying obsessional doubt that harm, illness, disorder or bad luck, will occur if compulsive actions are not carried out to neutralize the risk. Treatments of choice include pharmacotherapy and cognitive behavior therapy (CBT) either alone or in combination. Although these interventions are clinically effective, they usually do not reduce the obsessions to a sub-clinical level. There is a risk of relapse and even so a sizeable minority of people with OCD refuse or abandon treatment, and some specific subtypes of OCD show no or minimal response. CBT-as-usual, involves principally the administration of exposure and response prevention (E/RP). A cognitive psycho-education element may by added in line with current cognitive thinking that it is the appraisal the person attributes to the initial intrusive thought with engenders obsessional distress. Such cognitive treatment will focus on modifying the anxiety and beliefs about consequences following on from the initial obsessional preoccupation. A competing cognitive model termed the inference-based approach (IBA) developed by the principal investigator and co-workers offers an account of obsessions which focuses on the reasoning producing the initial obsessional doubt as the source of the OCD. In this model the obsessing begins at the point where the person crosses over from reality into the initial obsessional doubt since the doubt is not based on real perception but on a subjective narrative misleading the person to infer doubt where no doubt is required. Over the last 10 years, experimental, clinical and psychometric research studies (independently replicated) have supported the essential claims of the IBA and have led to development of inference-based therapy (IBT). A small scale randomized clinical trial, and clinical case studies have established IBT as a 'promising' intervention (according to American Psychological Association criteria) and shown that IBT seems to offer advantages over conventional CBT, particularly for treatment-resistant cases who over-invest in their obsessional convictions. The current randomized clinical trial will compare IBT to CBT as usual, and to a third active control condition of mindfulness. Mindfulness is a generic treatment shown effective in a range of psychiatric problems which seeks to calm thoughts by redirecting mental focus through meditative distancing. The mindfulness condition is a non-specific cognitive control for any confounding effect of mood change on symptom relief. CBT follows an 'appraisal' model either through restructuring appraisals and beliefs about the obsession or by de-dramatizing anxious expectations through exposure and reality testing (combined with E/RP). IBT seeks to change the reasoning leading up to the initial inference of doubt on the principle that eliminating the doubt logically and necessarily eliminates all anticipated consequences and accompanying obsessional anxiety. All three treatments will be manualized and administered by experienced professional therapists supervised by a co-investigator with an allegiance to one of the three approaches. The three treatment models operate on distinct processes and on separable stages of the obsessional chain, so therapy components will not overlap. The principal sites chosen will be Montreal Louis-H. Lafontaine Hospital and Gatineau Pierre-Janet Hospital. A total of 80 participants will be recruited over a five-year period into each treatment modality, giving a total of 240, which will permit robust conclusions even with weak effect sizes. Outcome evaluators will be blind to treatment modality and independent of therapists. All therapy will be delivered on an individual weekly basis until significant clinical improvement or up to a maximum of 26 weeks with a 6-month follow-up post-treatment. All parameters of treatment delivery will be identical across treatments. All sessions will be audio recorded and treatment integrity will be rated independently. Outcome measures will be standard DSM-IV based clinical diagnostic scales and also psychosocial measures of end-point functioning across life domains. Treatment process variables will be continuously monitored. Refusal and attrition are calculated at 16% and analysis will include intention to treat. The hypotheses are that: (1) all treatments will be effective in improving mood and reducing symptoms; (2) the IBT will be superior in terms of: (a) its consistent impact over all types of OCD; (b) its lower rate of relapse. The RCT will be the first of its kind in comparing these three active treatments and will impact on treatment of choice for OCD.