We have obtained long-slit spectra of NGC 253 in the J, H, K, and N bands, broadband images in the J, H, and bands, narrowband images centered at the wavelengths of Brc and 0) S(1), and K s H 2 (1, imaging spectroscopy centered on [Ne II] (12.8 km). We have subtracted a composite stellar spectrum from the galaxy spectrum to measure faint emission lines that otherwise would be buried in the complicated continuum structure. We use these data and data from the literature in a comprehensive reassessment of the starburst in this galaxy. We conÐrm that the [Fe II] emission is predominantly excited by supernova explosions and show that the rate of these events can be derived from the strength of the infrared [Fe II] lines. Although the emission superÐcially resembles a thermally excited spectrum, H 2 most of the infrared luminosity is excited by Ñuorescence in low-density gas. We conÐrm the presence H 2 of a bar and also show that this galaxy has a circumnuclear ring. The relation of these features to the gaseous bar seen in CO is in agreement with the general theoretical picture of how gas can be concentrated into galaxy centers by bars. We derive a strong upper limit of D37,000 K for the stars exciting the emission lines. We use velocity-resolved infrared spectra to determine the mass in the starburst region. Most of this mass appears to be locked up in the old, preexisting stellar population. Using these constraints and others to build an evolutionary synthesis model, we Ðnd that the initial mass function (IMF) originally derived to Ðt the starburst in M82 also accounts for the properties of NGC 253 ; this IMF is similar to a modiÐed Salpeter IMF. The models indicate that rapid massive star formation has been ongoing for 20È30 million yr in NGC 253 ; that is, it is in a late phase of its starburst. Its optical spectrum has characteristics of a transitional H II/weak-[O I] LINER. We model the emission-line spectrum expected from a late phase starburst and demonstrate that it reproduces these characteristics.

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