Ataxia-Telangiectasia (A-T) is a very rare multisystem disease of DNA repair, associated with progressive disabling neurological symptoms, respiratory failure, immunodeficiency and cancer predisposition, leading to premature death. There are no curative treatments available for A-T but clinical trials have begun. A major limiting factor in effectively evaluating therapies for A-T is the lack of suitable outcome measures and biomarkers. We have performed a systematic review to collect the information currently available on biomarkers for A-T both in patients and preclinical studies. We have identified 56 reports discussing potential A-T biomarkers in both pre-clinical models and patients. These studies report on diagnostic biomarkers but prognostic biomarkers and responsive markers of clinical status are currently lacking. Some biomarkers of neurodegeneration in A-T show promise, including non-invasive neuroimaging biomarkers. Some biomarkers of oxidative stress and responsive markers to radiotherapy and steroid treatment have potential value in clinical trials. The formation of the A-T biomarker working group with international experts is an important step forward to facilitate the sharing of materials, data and expertise with the common goal of finding effective biomarkers for A-T.