**Abstract**

This chapter will provide a brief overview of the fundamentals of photodynamic therapy with an emphasis on its use in a clinical setting. Beginning with the history and fundamental science underlying photodynamic therapy and delving into clinical uses. There will be a primary focus on understanding the use of photodynamic therapy under currently approved clinical indications along with their limitations. There are a number of approved therapeutic indications for photodynamic therapy, but there are important limitations and contraindications when applying this therapy. Photodynamic therapy, as applied to the clinical treatment of cancer will be the primary focus with further emphasis on endoluminal and specifically endobronchial cancer as the primary case study.

**Keywords:** photodynamic therapy, lung cancer, esophageal cancer, Barrett's esophagus, interventional pulmonology, interventional gastroenterology
