This paper presents a detailed calculation of infrared (IR) emission spectra from interstellar dust heated by starlight, focusing on mixtures of amorphous silicate and graphitic grains, including varying amounts of polycyclic aromatic hydrocarbon (PAH) particles. The models are constrained to reproduce the average Milky Way extinction curve. The calculations include the effects of single-photon heating and updated IR absorption properties for PAHs, which are consistent with observed emission spectra, including those obtained by the *Spitzer Space Telescope*. The authors find a size distribution for PAHs that gives emission band ratios consistent with observed spectra of the Milky Way and other galaxies. They present emission spectra for a wide range of starlight intensities and calculate how the efficiency of emission into different IR bands depends on PAH size. The strong 7.7 μm emission feature is primarily produced by PAH particles containing fewer than 10³ carbon atoms. The emission spectrum also depends on the starlight intensity relative to the local interstellar radiation field (U). The submm and far-infrared emission is compared to observed emission from the local interstellar medium. Using a simple distribution function, the authors calculate the emission spectrum for dust heated by a distribution of starlight intensities, such as occurs within galaxies. The models are parameterized by the PAH mass fraction (qPAH), the lower cutoff (Umin) of the starlight intensity distribution, and the fraction (γ) of the dust heated by starlight with U > Umin. They present graphical procedures using *Spitzer* IRAC and MIPS photometry to estimate the parameters qPAH, Umin, and γ, the fraction (fPDR) of the dust luminosity coming from photodissociation regions with U > 100, and the total dust mass (Mdust). The results are discussed in the context of interpreting IRAC and MIPS observations.This paper presents a detailed calculation of infrared (IR) emission spectra from interstellar dust heated by starlight, focusing on mixtures of amorphous silicate and graphitic grains, including varying amounts of polycyclic aromatic hydrocarbon (PAH) particles. The models are constrained to reproduce the average Milky Way extinction curve. The calculations include the effects of single-photon heating and updated IR absorption properties for PAHs, which are consistent with observed emission spectra, including those obtained by the *Spitzer Space Telescope*. The authors find a size distribution for PAHs that gives emission band ratios consistent with observed spectra of the Milky Way and other galaxies. They present emission spectra for a wide range of starlight intensities and calculate how the efficiency of emission into different IR bands depends on PAH size. The strong 7.7 μm emission feature is primarily produced by PAH particles containing fewer than 10³ carbon atoms. The emission spectrum also depends on the starlight intensity relative to the local interstellar radiation field (U). The submm and far-infrared emission is compared to observed emission from the local interstellar medium. Using a simple distribution function, the authors calculate the emission spectrum for dust heated by a distribution of starlight intensities, such as occurs within galaxies. The models are parameterized by the PAH mass fraction (qPAH), the lower cutoff (Umin) of the starlight intensity distribution, and the fraction (γ) of the dust heated by starlight with U > Umin. They present graphical procedures using *Spitzer* IRAC and MIPS photometry to estimate the parameters qPAH, Umin, and γ, the fraction (fPDR) of the dust luminosity coming from photodissociation regions with U > 100, and the total dust mass (Mdust). The results are discussed in the context of interpreting IRAC and MIPS observations.