Abstract: Following cell death, DNA can be released into the blood plasma and other body fluids in the form of cell-free DNA (cfDNA). These DNA fragments are typically ~167 bp in length, corresponding to the length of DNA wrapped around one nucleosome core (147 bp), plus DNA tails that survive endogenous DNase digestion (~10 bp). CfDNA is derived from a variety of sources, each having unique diagnostic applications. During pregnancy, fetal-derived cfDNA within an expectant mother’s blood plasma can diagnose fetal genetic abnormalities and is a well-established method for non-invasive prenatal testing. Cancer-derived cfDNA is also detectable within blood plasma by high sensitivity methods (e.g., NGS, ddPCR) and can accurately diagnose cancers by a minimally invasive blood test (Diehl et al., 2008). The short half-life of cfDNA (< 2 hours) also facilitates real-time measurement of disease burden, such as monitoring cancer relapse. There is now growing evidence of brain-derived cfDNA (bd-cfDNA) within the cerebrospinal fluid (CSF) and blood plasma. In this perspective, we review the present understanding, current challenges, and potential utility of bd-cfDNA.