H-alpha emission-line fluxes have been measured for all of the detected H II regions in 30 nearby spiral and irregular galaxies, and these data are used to study the behavior of the H II region luminosity functions (LFs). In most galaxies, the LF is well represented by a power law function. Both the normalization and the shape of the LF change systematically with Hubble type. Early-type spirals possess fewer H II regions of all luminosities, a steeper LF, and in many cases an upper cutoff or turnover in the LF. Taken together, these effects account for the virtual absence of 'supergiant' H II regions such as 30 Doradus in Sa-Sb galaxies. Most of the massive stars in late-type spiral and irregular galaxies form in large H II/OB complexes, while in early-type spirals the bulk of the massive star formation occurs in small regions ionized by no more than a few OB stars. 59 references.