  We discuss the various definitions of time correlation functions and how to
estimate them from experimental or simulation data. We start with the various
definitions, both in real and in Fourier space, and explain how to extract from
them a characteristic time scale. We then study how to estimate the correlation
functions, i.e.\ how to obtain a good approximation to them from a sample of
data obtained experimentally. Finally we discuss some practical details that
arise in the actual computation of these estimates, and we describe some
relevant algorithms.
