This chapter discusses the need for an FGS system along with an introduction to the fire and gas (F&G) detection system, challenges in the process plants, and causes of fire. This chapter also elaborates the industry safety performance standards and the critical components of F&G systems. There are various detectors used in fire and gas subsystems and some of the critical detectors along with their operating principles are also discussed. The F&G systems architecture and critical systems and subsystems that get integrated into the F&G network are discussed in detail. The chapter provides the details on typical implementation of such systems for a successful design.