The paper reviews metrology aspects of terahertz time-domain spectroscopy. Part one discusses noise and its effects on system performance, including frequency-dependent dynamic range and signal-to noise ratio, and the measurement bandwidth. Part two addressed sources of uncertainties and measurement errors arising from system design and from interactions with the object under test. Part three describes techniques of system calibration. Commonly encountered issues are addressed and simple practical solutions suggested. Existing literature on the subject is reviewed.