Phenothiazine and its derivatives have attracted a noteworthy attention within the medicinal chemistry' domain ascribed to its varied pharmacological activities and possible medically-beneficial applications. Fused S-heterocycle Phenothiazine being the most promising derivatives among all has displayed and showcased promising results in numerous biological evaluations. Phenothiazines are one of the class of compounds categorized by tricyclic structure which is comprised of central thiazine ring merged with two benzene rings. Such central thiazine rings incorporate the presence of sulphur atoms which acts as a catalyst to undergo various substitutions on the benzene rings that eventually results in versatility of the phenothiazines. The chapter focuses on the holistic understanding of the biological relevance of fused s-heterocycle phenothiazine derivatives and would highlight and elucidate the potential of phenothiazine derivative's development as therapeutic agent.