  These lecture notes provide a brief introduction to methods used to search
for a stochastic background of gravitational radiation -- a superposition of
gravitational-wave signals that are either too weak or too numerous to
individually detect. The focus of these notes is on relevant data analysis
techniques, not on the particular astrophysical or cosmological sources that
are responsible for producing the background. The lecture notes are divided
into two main parts: (i) an overview, consisting of a description of different
types of gravitational-wave backgrounds and an introduction to the method of
cross-correlating data from multiple detectors, which can be used to extract
the signal from the noise; (ii) details, extending the previous discussion to
non-trivial detector response, non-trivial overlap functions, and a recently
proposed Bayesian method to search for the gravitational-wave background
produced by stellar-mass binary black hole mergers throughout the universe.
Suggested exercises for the reader are given throughout the text, and compiled
in an appendix.
