Psoriasis is a complex disease. Dermatologists have not documented psoriasis severity, except in clinical trials; doing so requires tools for assessing psoriasis and an understanding of what changes in those assessments mean in terms of outcome. Two psoriasis assessment tools have dominated: The Psoriasis Area and Severity Index and the Dermatology Life Quality Index. There are advantages and disadvantages to each. Newer instruments may not be more suitable for documenting psoriasis. There may be benefits in terms of patient ownership of disease management from using self-assessment tools for documenting severity, for example, the Self-assessment version of the Simplified Psoriasis Index.