The emergence of microbial resistance to many of the major classes of antimicrobial drugs is becoming an increasing problem in the clinic and environmental areas. As such, the discovery of drugs with novel modes of action will be vital to meet the threats created by the emergence of resistance. Photodynamic antimicrobial chemotherapy is receiving considerable attention for its potentialities as a new form of antimicrobial treatment. This chapter will focus on the state of porphyrinic derivatives as antimicrobial agents in the clinic and environmental areas and will discuss some factors determining microbial inactivation efficiency by porphyrins.