  The energy of a finite system thermally connected to a thermal reservoir may
fluctuate, while the temperature is a constant representing a thermodynamic
property of the reservoir. The finite system can also be used as a thermometer
for the reservoir. From such a perspective the temperature has an uncertainty,
which can be treated within the framework of estimation theory. We review the
main results of this theory, and clarify some controversial issues regarding
temperature fluctuations. We also offer a simple example of a thermometer with
a small number of particles. We discuss the relevance of the total observation
time, which must be much longer than the decorrelation time.
