The excitonic insulator (EI) is a charge insulating state that arises from the spontaneous formation of bound electron-hole pairs (excitons). It presents an interesting platform for realizing quantum many-body ground states of bosons in solids, such as condensate and superfluid. Although the concept has been known for sixty years, to date the EI remains poorly established because reservoirs for excitons generally do not exist. In this talk, I will discuss the realization of an EI ground state with strongly interacting excitons in atomic double layers of transition metal dichalcogenide (TMD) semiconductors. The double layer system enables the formation of electrical reservoirs for interlayer excitons and the continuous tuning of the exciton chemical potential. I will present direct thermodynamic evidence of the EI state based on capacitance measurements. I will also discuss the exciton phase diagram that reveals both the exciton Mott transition and exciton quasi-condensation.