Seismometry is the technical discipline concerned with the detection and measurement of seismic ground motion. It comprises the design of seismographs, their calibration, their installation, and the quantitative interpretation of seismograms in terms of ground motion. A seismograph basically consists of one or more seismometers, a clock or time-signal receiver, and a recorder. The present article concentrates on the theory and usage of seismometers. It does not describe their technical design in detail, but mentions some general design principles. Emphasis is put on precise methods for calibrating and testing. Although most commercial seismometers are well calibrated and normally need not be recalibrated by the user after installation, such methods are essential to ensure that an instrument is in good working condition.