Measurements of the width or duration of waves in the ECG are widely used to define abnormal electrical conduction in the heart, to detect myocardial damage, and to stratify patients at risk of cardiac arrhythmias[1,2]. A number of automated methods for making such measurements, especially of the QT interval, have been designed. Several studies have been undertaken to evaluate the performance of such algorithms, but they have been hampered by a lack of standardized databases containing a sufficiently large number of carefully annotated heartbeats with manually-made measurements of waveform boundaries. This situation probably reflects the tremendous effort required for a clinician to manually annotate a statistically significant set of QRST complexes. We have attempted to address this problem by constructing an annotated reference database (the QT Database) which includes a wide variety of ECG morphologies, and a significant number of patient records. This paper will describe the design and construction of the QT Database.