Details are presented of an experimental and theoretical investigation of the defect sizing capabilities of the transient thermography technique. Back-drilled hole artificial defects, of diameters from 2 to 10 mm and at depths between 0.5 and 1.7 mm, in bakelite plates, were thermographically imaged. Full-width at half maximum (FWHM) of a defect image is shown to be accurately related to defect diameter. These measurements are also shown to be in excellent agreement with finite-difference simulations of the response of this type of defect. The temporal dependences of the images are shown to provide an indication of defect depth.