Since Johnson's experimental observations of thermal noise in 1927, an
d Nyquist's explanation of the phenomenon shortly afterwards in 1928,
thermal noise has attracted interest as a means of measuring temperatu
re, The independence of the thermal noise from the material nature of
the sensor makes it particularly attractive for metrological applicati
ons, However, the noise signals are extremely small and some ingenuity
is required to make accurate measurements, This paper reviews the fou
ndations of Johnson noise thermometry and the various techniques that
have been employed to measure temperature via Johnson noise, Emphasis
is placed on key developments in noise thermometers for metrological a
pplications, The review includes the current activities of teams invol
ved in noise thermometry research.