"A theoretical derivation of two-photon absorption (2PA) from bulk, surface and 2 dimensional (2D) semiconductors, based on the length gauge analysis and the electron density operator, is formulated; the intraband r_i part and the interband r_e part of the position operator r are properly accounted for. Within the independent particle approximation, the nonlinear third order susceptibility tensor χ abcd (−ω; −ω, −ω, ω) and the two-photon absorption coefficient are calculated, including the scissors correction needed to correct the well-known underestimation of the local-density-approximation band gap. Using time-reversal symmetry, it is shown that the expression for χ abcd (−ω; −ω, −ω, ω) is finite at ω = 0, avoiding nonphysical divergences presented in previous calculations when ω → 0. Ab initio band structure calculations using different pseudopotential schemes that include spin-orbit coupling are used to calculate the 2PA for several semiconductors, and the calculations are compared with available experimental results for photon energies that are below the optical band gap."