A psoriatic arthritis (PsA) module was convened at OMERACT 8 in order to achieve consensus on the core domains that should be included in randomized controlled trials and longitudinal observational cohorts of subjects with PsA. Following a plenary session at which current status of measures used to assess PsA were reviewed, and discussion at breakout groups, the group achieved consensus on 6 core domains: peripheral joint activity, skin activity, pain, patient global assessment, physical function, and health-related quality of life. In addition the following domains were considered important but not mandatory: spinal disease, dactylitis, enthesitis, fatigue, nail disease, radiography, physician global assessment, and acute-phase reactants. A research agenda was proposed to include development and validation of instruments for the domains where none existed, and in particular further research was recommended for the following areas: magnetic resonance imaging and ultrasound of joints, enthesitis, skin and synovial tissue analysis, and "participation."