Polycyclic aromatic hydrocarbons (PAHs) are the leading contenders for producing the strong IR emission bands observed between 3 and 20 microns in nearly all interstellar, circumstellar, galactic, and extragalactic environments. The interstellar PAH emission features exhibit significant variability that is dependent on several parameters. For example, the strength and shape of the radiation field, metallicity, etc. These variations reflect both the physical conditions in the emission zones, as well as, the composition of the PAH population in terms of structure, size, and charge, each responding differently to changes in the local environment. Here we present an overview of the contribution of relevant PAH populations to the emission in class A and B objects. We also provide quantitative relations for PAH size determination depending on the ionization fraction of the PAHs and the radiation field they are exposed to. In the dense ISM, PAHs freeze out on ice-covered dust grains and undergo chemical processing by UV photons and cosmic rays. Laboratory experiments have shown that the PAH evolution and chemical processing depend on chemical concentration, temperature, UV and cosmic fluxes, dust grain structure, and composition. Ab initio calculations allow for the identification of the irradiation products and to gain insight into the reaction mechanisms. Here we present the results of our synergistic laboratory and theoretical approach to study the chemical and spectroscopic characteristics of PAHs processed in interstellar ices. We also assess the contribution of neutral PAHs and hydrogenated PAHs in the Young Stellar Object (YSO) Mon R2 IRS 3 and the Taurus dense cloud toward Elias 16.