The diagnosis of arthrosis of the knee is based on anamnestic, clinical and radiological findings. Epidemiological Danish data are presented with a diagnostic algorithm. Treatment ranges from initially conservative – although with limited evidence of cost-benefit efficiency – towards surgical intervention. As opposed to dogma suggesting a delay of arthroplasty, early surgical intervention results in improved outcomes. Optimal treatment of co-morbidities and control of haemoglobin before referral to surgery optimizes patient outcome.