The article provides an overview of the significant advancements in the field of two-component signal transduction (TCST) since its discovery in 1986. The authors highlight the landmark publications that marked the beginning of this research, including the identification of amino acid sequence similarity and protein phosphorylation in these systems. The prototypical TCST consists of a sensor kinase and a response regulator, with the sensor kinase detecting environmental stimuli and phosphorylating the response regulator, which then controls the output domain to generate an adaptive response. Despite extensive research, many aspects of TCST remain poorly understood, such as the interactions between domains and auxiliary proteins, circuit architecture, and subcellular localization. The article is organized into sections covering sensor kinases, response regulators, phosphatases, networks, biological significance, and methods, each providing insights into specific aspects of TCST. The authors also discuss the phyletic distribution, evolutionary origins, and potential applications of TCST in synthetic biology and drug development. The article emphasizes the ongoing vitality and rapid progress in the field, driven by technological advancements and new experimental methods.The article provides an overview of the significant advancements in the field of two-component signal transduction (TCST) since its discovery in 1986. The authors highlight the landmark publications that marked the beginning of this research, including the identification of amino acid sequence similarity and protein phosphorylation in these systems. The prototypical TCST consists of a sensor kinase and a response regulator, with the sensor kinase detecting environmental stimuli and phosphorylating the response regulator, which then controls the output domain to generate an adaptive response. Despite extensive research, many aspects of TCST remain poorly understood, such as the interactions between domains and auxiliary proteins, circuit architecture, and subcellular localization. The article is organized into sections covering sensor kinases, response regulators, phosphatases, networks, biological significance, and methods, each providing insights into specific aspects of TCST. The authors also discuss the phyletic distribution, evolutionary origins, and potential applications of TCST in synthetic biology and drug development. The article emphasizes the ongoing vitality and rapid progress in the field, driven by technological advancements and new experimental methods.