  X-ray observations have played a key role in the study of substructure and
merging in galaxy clusters. I review the evidence for cluster substructure and
mergers obtained from X-ray observations with satellites that operated before
Chandra and XMM. Different techniques to study cluster mergers via X-ray
imaging and spectral data are discussed with an emphasis on the quantitative
analysis of cluster morphologies. I discuss the implications of measurements of
cluster morphologies for cosmology and the origin of radio halos.
