Liquid biopsy was introduced to the oncology field with the promise of revolutionizing the management of cancer patients, minimizing the exposure to invasive procedures such as tissue biopsy, and providing reliable information regarding therapy response and detection of disease relapse. Despite the significant increase in the number of published studies on circulating tumor DNA (ctDNA) in the past years, the emphasis of most studies is on the development of new technologies or on the clinical utility of ctDNA. This leaves a clear gap of knowledge concerning the biology of ctDNA, such as the fundamental mechanisms through which DNA from tumor cells is released into the circulation. Moreover, considering that ctDNA analysis is now currently being applied in clinical practice, the need for rigorous quality control is arising, and with it the necessity to standardize procedures, from sample collection to data analysis. This review focuses on the main aspects of ctDNA, including approaches currently available to evaluate tumor genetics, as well as the points that still require improvement in order to make liquid biopsy a key player in precision medicine.