The Harvard-Emory ECG Database (HEEDB) is a large collection of 12-lead electrocardiogram (ECG) recordings, developed through a collaboration between Harvard University and Emory University. The database consists of 10,608,417 unique ECG recordings from 1,818,247 patients from Massachusetts General Hospital (MGH) and 1,452,964 recordings from 552,481 patients from Emory University Hospital (EUH) collected in clinical settings as part of routine patient care since the early 1990s. Continuously updated with new data, the recordings consist of 10-second, 12-lead ECGs sampled at 250 and 500 Hz, and stored in WFDB format. Future updates will include demographic information such as age, sex, race, and ethnicity. Additionally, 12SL annotations-encompassing ECG diagnoses, morphology, and rhythms-are available for 10,471,531 recordings from MGH and 1,268,277 recordings from EUH. Shortly, ICD-9/10 codes, CPT codes, and medication history will also be made publicly available.