This book deals with the toxic chemical sensor applications of polypyrrole, which was functionalized with different organometallic species as the active material. The active species or the secondary dopant contains a central iron atom in different oxidation states to compare the sensing behavior of the materials. This book gives insight in the area of toxic gases, such as carbon monoxide in very low concentration. It also helps the researchers how to manipulate the material in order to get maximum sensitivity in shortest time. The chemical as well as electrochemical methods were adopted and described in detail for each species separately. The mechanistic aspects of the sensor material are correlated with structural features and conductivity studies of the modified polypyrroles. One can easily learn the various characterization techniques as it is explained widely in this book. The excellent stability, response time, and reproducibility of the sensor materials presented in this book may be very much helpful for further research and exploitation of this field. The sensor assembly is very simple and avoids any kind of complicated electronic and mechanical circuits.