  X-ray observations of the hot intra-cluster medium (ICM) in galaxy groups and
clusters provide quantities such as their gas mass, X-ray luminosity, and
temperature. The analysis of the scaling relations between these observable
properties gives considerable insight into the physical processes taking place
in the ICM. Furthermore, an understanding of the scaling relations between ICM
properties and the total cluster mass is essential for cosmological studies
with clusters. For these reasons, the X-ray scaling relations of groups and
clusters have been a major focus of research over the past several decades. In
this Chapter, after presenting the expectations from the self-similar model,
based on the assumption that only gravity drives the evolution of the ICM, we
discuss how the processes of gas cooling and non-gravitational heating are
believed to be responsible for the observed deviations from the self-similar
scenario. We also describe important complications that must be considered when
measuring and interpreting the scaling relations.
