Allogeneic hematopoietic stem cell transplantation (HSCT) is a life-saving treatment for people with severe blood-related diseases. However, it comes with serious risks, including a condition called graft-versus-host disease (GVHD), where the transplanted cells attack the patient's body. GVHD can occur in about 50% of patients acutely and 35% in a chronic form, potentially affecting organs like the skin, liver, and gastrointestinal system. Currently, doctors diagnose GVHD based on symptoms, as there are no easy tests available. Infections can also be a problem after HSCT, as dormant viruses may reactivate. These infections are monitored using specialized tests. Additionally, doctors use advanced methods, like analyzing minimal residual disease (MRD) and chimerism, to check for the risk of the original disease coming back. MRD is tracked by looking for specific genetic markers of the disease in the patient's blood or bone marrow. Another emerging tool involves analyzing cell-free DNA (cfDNA)-tiny fragments of DNA found in bodily fluids that come from dying cells. This technique, called liquid biopsy, has been revolutionary in areas like cancer detection, pregnancy testing, and organ transplants. For example, in organ transplants, cfDNA can indicate early signs of rejection, helping reduce the need for invasive biopsies. In HSCT, the use of cfDNA to monitor complications like GVHD or relapse has not been fully explored. This pilot study aims to investigate whether analyzing cfDNA using a technique called epigenomic profiling can help detect acute GVHD, as well as other post-transplant issues like infections, disease relapse, and chronic GVHD. The goal is to compare cfDNA analysis to current testing methods to see if it offers better or earlier detection of complications. This research could pave the way for improved, less invasive monitoring of HSCT patients, potentially leading to better outcomes and fewer complications.