Degraded skin conditions and advanced forgery technologies have reduced the stability and security of identity authentication based on external biometrics of the fingertip, including fingerprints, sweat pores, etc. Biological studies indicate that these external biometrics have their subcutaneous origins. Following the successful application of optical coherence tomography (OCT), the fingertip subcutaneous biometrics such as internal fingerprints and sweat glands have been collected. While researchers have worked on different aspects on subcutaneous biometrics, there is a lack of review of the methods and applications to our best knowledge. A survey on every key step in the subcutaneous biometrics processing is conducted in this paper, including the OCT system design, the extraction of subcutaneous biometrics, the identification and anti-counterfeiting applications of new features. Discussion on the pros and cons of methods, and analysis on the research and application directions, are made in every step. The linkage and difference with the conventional fingerprints are illustrated. Fingertip subcutaneous biometrics will undoubtedly become a new trend in highly secure authentication. This paper intends to provide systematic references and insights of methods and applications and support the researchers for their works on fingertip subcutaneous biometrics.