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Subsequent to the discovery of the infrared-luminous galaxies by the IRAS satellite in 1983, a great deal of research activity has been invested in efforts to understand the energy generation mechanisms powering these unusual objects. We have applied newly available optical emission line diagnostics in a program to discriminate between possible energy generation mechanisms, i.e., active nuclei and starbursts, in two samples of IRAS galaxies. The first sample was composed of very faint (F(60$\mu{\rm m}) <$ 180 mJy, ${\rm z} < 0.3$) galaxies drawn from coadded IRAS scans of the North Ecliptic Pole Region. We acquired spectra of 72 sources from the NEPR sample and mapped the redshift distribution of these objects. Of this sample 17 spectra were suitable for emission line analysis. While 3 sources could be classified as AGNs and another 7 as starburst galaxies, the remaining 7 eluded classification. The second sample consisted of 37 much brighter sources drawn from the IRAS Bright Galaxy Sample. Longslit optical emission-line spectra, which separated nuclear and disk contributions to the emission, indicated the BGS subsample contained 7 AGNs and 24 starbursts. In a minority of 6 cases, however, the line diagnostics failed to unambiguously classify the luminosity source. Follow-up near-infrared imaging and spectroscopy to characterize the age and distributions of the stellar populations of the 'straddlers' and mid-infrared imaging to constrain the sizes of the starburst disk components were performed to better understand why the optical diagnostics failed for some galaxies. The 'straddlers' optical and near-infrared characteristics seem to be consistent with unresolved low-luminosity active nuclei residing within a starburst disk.
