Strain-softening behavior is defined as a gradual loss of shear resistance after a peak strength has been reached. The basic concepts of the theories of elasticity and plasticity are reviewed, and the classical formulation is extended to include analysis of strain softening. A general effective stress-strain-strength model is described, but attention herein is focused on undrained behavior. Closed-form mathematical solutions for stresses, strains, and pore-water pressures around a pressuremeter are worked out, and detailed comparisons are made between the results from strain-hardening and strain-softening behavior. The proposed theory makes it possible to back-figure basic soil stress-strain and strength properties from strain-controlled pressuremeter tests.