Plaque psoriasis is a chronic, inflammatory skin disease characterized by the presence of erythematous inflammatory plaques. The plaques may be itchy or painful and are usually covered by silver, flaking scales. In addition to the overt dermatological symptoms, plaque psoriasis is often associated with psychosocial symptoms that can impact various aspects of social functioning including interpersonal relationships and performance at school or work. The estimated number of Canadians living with psoriasis is approximately one million and plaque psoriasis is the most common form, representing approximately 90% of cases. The severity of psoriasis is classified as mild, moderate, or severe using criteria such as body surface area (BSA) or scores on the Psoriasis Area and Severity Index (PASI) and Dermatology Life Quality Index (DLQI). The Canadian Guidelines for the Management of Plaque Psoriasis provide two definitions for each measure of disease severity: definitions used in clinical trials and definitions for clinical practice. In clinical trials, moderate-tosevere psoriasis is defined by a lower limit of 10% BSA and the same limits have been used to define severe psoriasis. The guidelines also highlight that despite the literature available, there is a lack of consensus regarding how disease severity should be defined. This was affirmed by the clinical expert on this review. In patients with plaque psoriasis, topical agents (such as corticosteroids, vitamin D3 analogues, and retinoids) are the most widely used treatments for the mild form of the disease. Combination therapy may also be considered, which is typically more efficacious than monotherapy. The Canadian guidelines define moderate-to-severe psoriasis, clinically, by the inability to be controlled by topical therapy; however, it also states that topical agents used to treat mild psoriasis may still be useful adjunct therapy to systemic therapies or phototherapy. According to the clinical expert consulted for this review, if adequate improvement cannot be achieved with topical therapy and/or phototherapy, the systemic therapies such as cyclosporine, methotrexate, or biologic agents are considered. With a variety of treatments available, the approach to treating plaque psoriasis is heavily patient-centred where the goals of therapy may differ from patient to patient. It is widely accepted that an effective treatment for plaque psoriasis is also one that a patient is willing to work with. The patient-centred approach was aligned with feedback from the clinical expert consulted for this review. The drug under review, halobetasol propionate (HP) and tazarotene (TAZ) lotion (Duobrii), is a combination product composed of a topical superpotent corticosteroid (0.01% weight by weight [w/w] HP) and a retinoid product (0.045% w/w TAZ). In Canada, HP/TAZ is indicated for improving the signs and symptoms of plaque psoriasis in adult patients with moderate-to-severe plaque psoriasis. It is recommended that HP/TAZ is applied in a thin layer to the affected area once a day, and the total dosage should not exceed 50 g per week. If no improvement is seen within 12 weeks of treatment, reassessment of the diagnosis may be necessary. The sponsor is requesting that HP/TAZ be reimbursed as per the indication reviewed by CADTH. The objective of this report is to perform a systematic review of the beneficial and harmful effects of 0.01% w/w HP and 0.045% w/w TAZ topical lotion for improving the signs and symptoms of plaque psoriasis in adult patients with moderate-to-severe plaque psoriasis.