The paper describes the theory and practice associated with the torsional quartz-crystal viscometer for the measurement of the viscosity of Newtonian Fluids. It is an instrument that has been less often used than its quality merits, but it has the distinct advantages, shared with the vibrating-wire device, that it involves no bulk motion of fluid or a solid and that all measurements can be electrical. The temperature range that can be covered by the instrument is from 2 to 650 K and pressures have reached as much as 100 MPa. The review summarizes the most recent theory of the instrument and carefully sets out all of the many conditions that have to be satisfied by design so that a practical instrument conforms to the theory. Most of the conditions are readily satisfied. Two working equations are presented that could be used to evaluate the viscosity using the frequency at resonance of the crystal and the bandwidth of that resonance when the crystal is immersed in the fluid and in vacuo. It is explained that at present only one of these equations should be used for the evaluation. Several configurations of instruments that have been employed for measurements over a wide range of conditions are briefly described as well as the corrections necessary to operate the instrument with the highest accuracy. The overall relative uncertainty attainable with the instrument ranges from 0.005 to 0.02 at a 95% confidence level, depending upon the fluid density.