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We have used a detailed non-LTE synthetic spectrum code to interpret RGO archive spectra from the nebular phase of SN 1993J and SN 1994I. Our SN 1993J calculations are based on a helium star explosion model from Nomoto \etal (1993), while our SN 1994I calculations are based on a C+O star model from Nomoto \etal (1994). Non-thermal energy deposition from radioactive decay of ${}^{56}{\rm Co}$ in the ejecta produces most of the emission during the first year of the nebular phase. However, after about 350 days, the SN 1993J spectrum shows strong H$\alpha$ emission associated with the circumstellar interaction. From our model calculations, we derive constraints on the mass and distribution of oxygen and helium and, for SN 1993J, constraints on the mass and distribution of hydrogen. We find that differences in the observed spectra of SN 1993J and SN 1994I can be directly related to the different core masses and expansion velocities of these supernovae. The evolution of the flux and width of the H$\alpha$ line in SN 1993J is a unique probe of the hydrodynamic structure, and an indicator of the soft X-ray flux produced by the interaction of the ejecta and the circumstellar medium. Self-consistent models of the radio, optical, and X-ray observations will be discussed.
