Increasing antibiotic resistant pathogens incidence brings to an end of “the antibiotic era” which extended over the past 50 years, and necessitates exploration of alternative approaches to combat emerging infections. Increase global spread of drug resistance pathogens has prompted researchers to search for new strategies for microbial eradication. The efficient and often selective inactivation of microbial pathogens by means of photosensitized processes (Photodynamic therapy) has opened favourable avenues to treat numerable infectious diseases. Antimicrobial Photodynamic therapy (APDT) is a light-based antimicrobial therapy capable of efficiently eradicating wide microorganisms. It is an oxygen-dependent photochemical reaction that occurs upon light mediated activation of a photosensitizing compound leading to the generation of reactive oxygen species. Antimicrobial photodynamic therapy is a topical, non-invasive approach suitable for treating local infections. This chapter focus on introduction to antimicrobial photodynamic therapy with an emphasis on the use of photodynamic therapy for the treatment of resistant microbial strains.