Field investigators have recently discovered thin, tabular zones of pure compressional deformation that they called compaction bands. These bands were found in association with shear bands and were postulated to be genetically related to them. At the laboratory scale, compaction bands have been noticed in association with boreholes and preexisting, artificial shear cracks subjected to compressive stress fields. Natural compaction bands are noticeable in outcrops because of their resistance to weathering; however, they may be more difficult to discern on freshly cut rock surfaces such as drill core or borehole walls. Because of the much reduced porosity in the compaction bands, these structures are potentially important as permeability barriers in reservoirs and aquifers in porous rocks. For bands associated with boreholes, the crushed material can be washed into the borehole contributing to sand production and possibly altering the stability of the borehole. This paper examines a theoretical framework that explains these features as a constitutive instability leading to localized compaction in a way completely analogous to shear strain localization. Conventional triaxial experiments on Castlegate sandstone resulted in compaction bands. In addition, thick deformation bands having normals at low angles to the maximum compression are present in some specimens. ¿ 1999 American Geophysical Union