Photodynamic therapy (PDT) utilizes photosensitizers (PS) together with irradiating light of specific wavelength interacting with oxygen to generate cytotoxic reactive oxygen species, in particular singlet oxygen, which has been approved for the clinical treatments of several malignant and non-malignant pathologies. Most recently, enhanced PDT was successfully achieved by using new light sources (i.e., light-emitting diode), novel functional nano-PSs, oxygen supply and synergistic therapy. In addition, PDT was widely used in the fields of antibacterial, antimicrobial and antibiofilm activities. The aim of this book is to highlight innovations in the fundamental mechanisms of enhanced PDT for clinical applications, and 20 papers published in the Journal of Innovative Optical Health Sciences during 2020-2023 were selected.