We have initiated a study of atomic scale defects which may play important roles in the reliability physics of a leading high dielectric constant/Si system: atomic layer deposition (ALD) HfO2 on silicon. We have utilized capacitance versus voltage and electron spin resonance measurements to explore electrically active defects in ALD HfO2/Si device structures. We have subjected some of these structures to either vacuum ultraviolet (VUV) illumination or gamma irradiation. The VUV illumination and gamma irradiation flood the dielectric with electrons and holes. Post irradiation measurements most strongly indicate the presence of high densities of large capture cross section electron traps. Electron spin resonance measurements clearly indicate the presence of silicon dangling bond interface defects which are similar to but not identical to the silicon dangling bonds observed at conventional Si/SiO2 interfaces.