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Photodissociation regions (PDRs) are predominantly neutral regions of the ISM in which the heating and chemistry are mainly regulated by far ultraviolet photons. They are extended regions at the interface between bright stars and molecular clouds comma and contain dense structures and clumps immersed in a more diffuse medium which are subjected to photo-evaporation comma which brings fresh matter into the diffuse hotter zone. The interaction of stellar radiation with in situ material includesdoublePoint 1) the disruption of grain mantles-clusters and coagulated grains formed in shielded dense regions comma 2) ionization and dissociation of the gas and 3) gas and dust heating. Studies of nearby PDRs have shown that these processes are strongly stratified and active on angular scales that can be as small as virgul1d_comma (0.002 pc-400 au at a distance of 400 pc) comma indicating that the physical conditions vary over orders of magnitude on small spatial-scales in PDRs. Nearby PDRs are therefore unique targets to study rapid variations in the dust and gas components as a function of the excitation and physical conditions. In light of the importance of PDRs in understanding ISM physics and chemistry comma we propose to use STIS on HST to study two emblamematic PDRs comma the Horsehead and NGC7023 comma at wavelengths covering the dominant energy input into the PDR comma the optical and UV. These data will provide data that complements an accepted JWST GTO program combining imaging and spectroscopy of both PDRs. We will observe regions matched to the apertures defined in the JWST program from virgul100-1000 nm using four STIS gratings comma two paired with the STIS MAMA detectors and two with the STIS CCD detectors comma all four in low resolution.