Results are presented for a study conducted at the NASA Langley Research Center which examined the effects of advanced technologies on the performance and size of very large, long-range subsonic transports. The study was performed using the Flight Optimization System (FLOPS), a multidisciplinary system of computer programs for conceptual and preliminary design and evaluation of advanced aircraft concepts. A four-engine, baseline configuration representative of existing transport technology was defined having a payload of 412 passengers plus baggage and a design range of 7300 nmi. New 600, 800 and 1000 passenger advanced transport concepts were then developed and compared to the baseline configuration. The technologies examined include 1995 engines, high aspect ratio supercritical wings, composite materials for the wing, fuselage and empennage, and hybrid laminar flow control (HLFC). All operational and regulatory requirements and constraints, such as fuel reserves, balanced field length, and second segment climb gradient were satisfied during the design process. Results indicate that near-term technologies offer the potential to design 600-passenger transports having takeoff gross weights no larger than that of the 412-passenger baseline configuration, but with available seat miles per gallon approximately 74% higher. The additional incorporation of more advanced technologies offers the potential for 600 passenger transports with significantly lower takeoff gross weights and available seat miles per gallon more than double that for the 412-passenger baseline configuration. Also, for the 600, 800 and 1000-passenger transport concepts, technology provides significantly larger gains in available seat mile per gallon than size increase alone.