Permeability versus pressure data for 27 tight shale and compacted seafloor mud samples (28 measurements), obtained from several sedimentary basins in North America by the Geological Survey of Canada, have been analyzed in order to determine the shale permeability-pressure characteristics. Tight shales are known to have some of the lowest permeabilities (10-23-10-19 m2) for rocks. Results indicate that these samples can be divided into three groups, AA, BB, and CC, on the basis of their permeability (k) versus effective pressure (Pe) characteristics. Group AA is characterized by relatively large k values (1.0x10-20-1.6x10-18 m²) that decrease at moderate to rapid rates with increasing Pe to 20-30 MPa where they level out. The group BB characteristics are similar, except that the k values are considerably smaller (1.0x10-22-1.0x10-19 m²). Group CC is characterized by large k values (3x10-19-1.6x10-18 m²) that decrease rapidly to very small k values (3x10-22- 1.6x10-21 m²) at Pe=60 MPa. Group CC may represent a texture with a weakened pore framework.