Recently, Transactive Energy Systems (TES) have gained great interest in the Power and Energy community. TES optimizes the operation of distributed energy resources (DERs) through market-based transactions between participants. The underlying transactive coordination and control (TC2) incorporates the economic concepts and principles into the decision making and controller design of individual market participants, and uses value, in particular, price as the control signal to coordinate and achieve their distinct objectives. It shares the advantages of both direct load control and price responsive control. However, because TES is a newly emerging concept, it is still mysterious to the power community. In this tutorial, pioneering TES researchers will give lectures to demystify TES from different aspects including theory, tool and application.