This chapter presents the Active Core Optical Fiber Chemical Sensor (AC-OFCS) techniques demonstrating the high sensitive nature of fiber-optic sensors. Fiber core optical absorption techniques have been proposed for monitoring high-energy radiation and ionization particles for applications in nuclear facilities. OFCS detects the existence of and measures the concentration of a compound in a sample through detecting the interaction of the compound with light propagating inside an optical fiber. Three types of special optical fibers, porous solid optical fibers, liquid-core waveguides, and hollow waveguides (HWG), have been used in designing AC-OFCS. Some very sensitive chemical sensors have been developed using these special optical fibers. The chapter also presents the sensor technologies demonstrated AC-OFCS's high sensitivity nature. Further development of AC-OFCS technologies largely depends on advances in other fields, such as material science, HWG, laser, and optoelectronics. An evanescent wave optical fiber chemical sensor can use conventional optical fibers made for communication industry to construct the sensor.