By providing direct, non-invasive visualisation of the microcirculation, nailfold capillaroscopy has potential as a biomarker in rheumatological diseases characterised by microvascular abnormality, especially systemic sclerosis and dermatomyositis. This chapter begins by describing the rationale for nailfold capillaroscopy as a biomarker, including reported associations between capillaroscopic findings and other markers of disease status. Technical challenges in image acquisition and interpretation (including imaging different skin tones) are then discussed, and reliability. Using capillaroscopy to monitor disease in the clinical setting is briefly described, before discussion of capillaroscopy as a tool (and possible future biomarker) to monitor disease in the research setting, including monitoring of treatment response (examples of these are given). Finally, ways in which future technologies (specifically the use of artificial intelligence) will facilitate development of capillaroscopy as a biomarker are summarised.