In older people, giant cell arteritis (also known as temporal arteritis) is a vision-threatening condition that can also cause strokes, heart attacks and other serious problems. This app is aimed at providing physicians with an evidence-based estimate of the probability of giant cell arteritis in a patient. Family doctors, internists, ophthalmologists, and neurologists can use it as a complement to – NOT a substitute for – their own clinical judgment, based on the input of age, symptoms, signs and laboratory tests. For illustrative purposes, the app also provides a hypothetical course of action, based on the probabilities derived from symptoms and signs, that was suggested by an algorithm created by Niederkohr and Levin (Management of the patient with suspected temporal arteritis, a decision analytic approach, Ophthalmology 2005; 48: 675-80). The interested physician should read this paper to decide the utility of this algorithm. Inclusion of this material is not an endorsement of the algorithm’s proposed course of action for a given patient: each case needs to be decided on its own merits. Remember too that the outcome of any estimating process like this is only as good as the data that you enter, in terms of both accuracy and completeness.