Talc occurs as a product of alteration of ultramafic rocks and may be a significant constituent of oceanic lithosphere and the arc mantle wedge. We explore the physical properties and structure of talc over a wide pressure range with density functional theory using the variable cell shape plane wave pseudopotential method in the local density (LDA) and generalized gradient (GGA) approximations. For LDA, the theoretical equation of state agrees with room temperature experimental measurements of the volume to within 2% when estimated effects of lattice vibrations are accounted for. We predict a spinodal mechanical instability (vanishing bulk modulus) at a volume 4% larger than the experimental volume at ambient conditions. Results for compression and tension are well represented by a fourth-order Birch-Murnaghan finite strain expansion with K0 = 37.8, K0' = 13. 6, K0K0″ = -153, where K is the bulk modulus, primes indicate pressure derivatives and subscript 0 refers to zero pressure. The estimated theoretical value of K0 at 300 K (29 GPa) is considerably less than that determined from the experimental equation of state, a difference we attribute to the constraints of strain compatibility in the polycrystalline laboratory sample. The c* = c sin ¿ axis is found to be a factor of 7 more compressible than the b axis, consistent with experimental data. Compression in the a--b plane is accommodated primarily by the Mg-octahedra. The polyhedral bulk modulus of the Mg-octahedra is considerably greater than one-third the linear modulus of the a axis. The difference is attributed to the increasing regularity of the Mg-octahedra on compression as measured by the octahedral tilt angle. The t sheet maintains registry with the o sheet by rotation of the Si-tetrahedra; the tetrahedral rotation angle rises from a value of 3¿ at the experimental volume to a theoretically predicted value of 22¿ at a volume of 170 ¿3 where the pressure is 26 GPa. Tetrahedral rotation leads to small Si-O-Si angles at the highest pressures investigated (<122¿ at 26 GPa), which leads us to speculate that talc may undergo pressure-induced amorphization under low-temperature compression.