We study a combined sample of 264 star-forming , 51 composite , and 73 active galaxies using optical spectra from SDSS and mid-infrared ( mid-IR ) spectra from the Spitzer Infrared Spectrograph . We examine optical and mid-IR spectroscopic diagnostics that probe the amount of star formation and relative energetic contributions from star formation and an active galactic nucleus ( AGN ) . Overall we find good agreement between optical and mid-IR diagnostics . Misclassifications of galaxies based on the SDSS spectra are rare despite the presence of dust obscuration . The luminosity of the [ NeII ] 12.8 \mu m emission-line is well correlated with the star formation rate ( SFR ) measured from the SDSS spectra , and this holds for the star forming , composite , and AGN-dominated systems . AGN show a clear excess of [ NeIII ] 15.6 \mu m emission relative to star forming and composite systems . We find good qualitative agreement between various parameters that probe the relative contributions of the AGN and star formation , including : the mid-IR spectral slope , the ratio of the [ NeV ] 14.3 \mu m to [ NeII ] \mu m 12.8 fluxes , the equivalent widths of the 7.7 , 11.3 , and 17 \mu m PAH features , and the optical “ D ” parameter which measures the distance a source lies from the locus of star forming galaxies in the optical BPT emission-line diagnostic diagram . We also consider the behavior of the three individual PAH features by examining how their flux ratios depend upon the degree of AGN-dominance . We find that the PAH 11.3 \mu m feature is significantly suppressed in the most AGN-dominated systems .