One of the most profound development in the contemporary high energy physics is the discovery of the AdS/CFT correspondence (or more generally the gauge-gravity duality), which states exact equivalence between a class of string theory vacua and certain quantum field theories. I will present the original Maldacena's argument for the existence of the AdS/CFT correspondence, as well as various qualitative and quantitative indications why this conjecture seems to be correct. If time permits, I will discuss recent research trends in this field.