In 1992, United States Department of Energy's (DOE) Advanced Turbine Systems (ATS) program established a target of 60% efficiency for utility scale gas turbine (GT) power plants to be achieved by the year 2000. Although the program led to numerous technology breakthroughs, it took another decade for an actual combined cycle (CC) power plant with an H class GT to reach (and surpass) the target efficiency. Today, another target benchmark, 65% efficiency, circulates frequently in trade publications and engineering journals with scant support from existing technology, its development path as well as material limits, and almost no regard to theoretical (e.g., underlying physics) and practical (e.g., cost, complexity, reliability, and constructability) concerns. This paper attempts to put such claims to test and establish the room left for gas turbine combined cycle (GTCC) growth in the next two decades. The analysis and conclusions are firmly based on fundamental thermodynamic principles with carefully and precisely laid out assumptions and supported by rigorous calculations. The goal is to arm the practicing engineer with a consistent, coherent, and self-standing reference to critically evaluate claims, predictions, and other futuristic information pertaining to GTCC technology.