Massive stars disrupt their natal molecular cloud material by dissociating molecules, ionizing atoms and molecules, and heating the gas and dust. Much of this interaction occurs in Photo-Dissociation Regions (PDRs) where far-ultraviolet (FUV) photons of these stars create a largely neutral, but warm region of gas and dust (Hollenbach et al. 1999). PDR emission dominates the infrared and sub-mm spectra of star-forming galaxies and also provides a unique tool to study in detail the physical and chemical processes that are relevant for most of the mass in inter- and circumstellar media.

I will review on recent high angular resolution observations in the far-infrared and sub-mm domains (Herschel, ALMA) that have challenged the traditional view of PDRs and unambiguously revealed a steeply varying interface between the molecular clouds and the ionized gas which betrays dynamic effects (e.g., Goicoechea et al. 2016, Joblin et al. 2018). Through some physical and chemical boundaries, one can start having access to the physical conditions and the chemical composition of a very structured medium with entangled dense molecular filaments and globules, and evaporation flows that advects material into the ionized region. Detailed analysis of the best spectral maps at small spatial scales in various tracers (high-J CO, H2, PAHs, dust) will be presented (e.g., Parikka et al. 2017, 2018, Schirmer et al. in prep.) and used to test the theoretical models (PDR Meudon code, DustEM/THEMIS). For prototypical PDRs with different excitation conditions (i.e., Horsehead, Orion Bar), how correlate the dust evolution with the changing physical conditions and vice-versa will be in particular investigated.

In the very near future, the James Webb Space Telescope (JWST) will observe in the near- to mid-infrared domains (over the full 0.6-28 μm range) with a sensitivity and a spatial resolution better than one to two orders of magnitude than its predecessors. JWST will resolve and directly observe, for the first time, the response of PDR gas to the penetrating FUV photons in its key zones (i.e. the H/H2 photodissociation front and the ionization front), where the main radiative heating and photochemical feedback processes occur. Understanding the intricate combination of physical, chemical and dynamical processes at the origin of the extremely rich IR spectra of PDRs is a key objective for JWST. In this context and in order to guide the preparation of the future proposals on star-forming regions in our Galaxy and beyond, I will present the ERS (Early Release Science, http://jwst-ism.org) and GTO (Guaranteed Time) programs on local PDRs and underline the important JWST-Herschel-ALMA synergy.