Porphyrins are macrocyclic aromatic compounds, which are similar to chlorophyll. They have high stability, high optical absorption and typical organic p-type semiconductor's behavior. This book is a summary of the porphyrin facts and can be used by students and beginner researchers. In the present book, I have brought the brief introduction and synthesis of porphyrin compounds, especially common tetra-cationic porphyrins and their applications in the photo-inactivation of bacteria. I hope this book will be useful for researchers.