Circulating cell-free DNA (cfDNA) and RNA (cfRNA) hold enormous potential as a new class of biomarkers for the development of non-invasive liquid biopsies in many diseases and conditions. In recent years, cfDNA and cfRNA have been studied intensely as tools for non-invasive prenatal testing, solid organ transplantation, cancer screening, and monitoring of tumors. In obesity, higher cfDNA concentration indicates accelerated cellular turnover of adipocytes during expansion of adipose mass and may be directly involved in the development of adipose tissue insulin resistance by inducing inflammation. Furthermore, cfDNA and cfRNA have promising diagnostic value in a range of obesity-related metabolic disorders, such as non-alcoholic fatty liver disease, type 2 diabetes, and diabetic complications. Here, we review the current and future applications of cfDNA and cfRNA within clinical diagnostics, discuss technical and analytical challenges in the field, and summarise the opportunities of using cfDNA and cfRNA in the diagnostics and prognostics of obesity-related metabolic disorders.