1. Introduction
2. Observations
2.1. Herschel Survey
2.2. ALMA Detection of the 203-506 Proplyd
3. 1D Model of the PDR in an Externally-Illuminated Protoplanetary Disk
4. Results of Proplyd Models
5. Effect of the H/H₂ Transition Layer Location on the Gas Thermal Balance at the Disk Surface
6. Photo Evaporation and Mass-Loss Rates
7. Conclusions