| Abstract | IRAS observations of 162 blazars are presented, with a detection rate of over 50%. The far-infrared data are combined with published data at other wavelengths to complete 93 energy distributions from 10^8^ to 10^18^ Hz. Far-infrared variability is found in ten out of 24 objects, with a typical amplitude of less than half the amplitude at optical wavelengths. Blazars are significantly more variable than quasars or Seyfert galaxies across the entire electromagnetic spectrum, with evidence for a single spectral component from millimeter to optical wavelengths. The range of variability in both spectral index and flux density increases with decreasing wavelength. A general excess of low-frequency emission may represent multiple radio components. The spectra of blazars steepen continuously between 10^9^ and 10^15^ Hz, and the x-ray flux density is often close to an extrapolation of the optical/ ultraviolet continuum. The wavelength at which the energy distribution turns down is ~1.5 mm, with a very narrow range of far-infrared spectral indices. Overall, there is a striking homogeneity in the spectral parameters of the most compact synchrotron component. Half of the blazars with total luminosities less than 10^12^ L_sun_ show evidence for thermal infrared components. Objects with emission lines have a stronger ultraviolet continuum bump than lineless objects. The global energy budget of blazars is dominated by infrared emission from 1 to 100 microns, and the bolometric luminosities range from 10^9^ to 10^14^ L_sun_. |
