When a child is diagnosed with cancer, the first step of the trajectory to recovery is often a surgical biopsy. The patient must undergo an invasive and risky procedure to take out (a piece of) the tumor and send it to the lab to gain detailed information about its identity. The doctors can then determine the optimal treatment plan to battle the cancer. However, this surgical biopsy is costly, dangerous and time-intensive, and is therefore not suited to monitor patients in the long term. Liquid biopsies make use of tumor material present in blood or other body fluids to follow tumor growth, remission, and emerging therapy resistance. Compared to tissue biopsies, liquid biopsies are easier to perform, are less costly and less dangerous, and can therefore be taken much more frequently. This makes them suited as a complementary technique to enable closer monitoring of a patient’s tumor and therefore detect tumor growth or therapy resistance much quicker, so that therapy adaptations can be made. These arguments combined contribute to the treatment, survival and quality of life of the patient. In addition, liquid biopsies will reduce the financial cost for society and lessen the burden carried by cancer patients and their loved ones.