A global optimization technique based on a genetic algorithm is proposed for microwave nondestructive evaluation. Starting from an integral formulation of the inverse scattering problem, the detection of a flaw in a known host medium is reduced to the minimization of a suitable nonlinear functional relating the measured field to the field predicted at a given iteration. The geometrical parameters of the flaw are retrieved by using a tomographic imaging approach. Numerical results are reported concerning cracks in lossless and lossy structures. The effects of the noise on measured input data are also analyzed.