Over the last twenty years, there has been growing evidence that our universe is dominated by dark energy. The nature of this dark energy remains a mystery. Is it the signature of the breakdown of general relativity or vacuum energy associated with quantum gravity? I will review the current observations and note the intriguing tensions between measurements based on the cosmic microwave background (CMB) and local measurements of the expansion rate of the universe and the amplitude of density fluctuations. I will then discuss on-going and upcoming CMB experiments and the role of the WFIRST mission in studying the nature of dark energy. I will also discuss the broader scientific mission of the WFIRST mission and its current status.