The original Shuttle Extravehicular Mobility Unit (EMU) Hard Upper Torso (HUT) configuration developed in1978 by Hamilton Standard and ILC, Dover had the arm attached in such a way that the shoulder bearing outer race was integral with the HUT. This method of attachment has been termed “planar arm.” During development, this configuration proved unacceptable because some astronauts and test subjects experienced difficulty, and in some cases pain, while donning. Interference occurred when the arms transitioned from vertical to horizontal as the HUT was entered (arms over head). At the time, designers needed to quickly resolve this issue and certify the EMU for the first Shuttle flight. The solution - pivot sockets - allowed the shoulder bearing to pivot relative to the HUT for donning purposes and then pivot back to allow for optimum arm performance.The pivoted HUT configuration has been very successful and is one of the design features that allows arm mobility and range in the EMU. However, this configuration has some limitations. There are life critical, single point failure modes associated with this design. Also, the tolerances and necessary inspection associated with the pivot configuration have impacted cost and schedule. In addition, the composite pivot/bellows configuration has useful-life limitations.In 1990, NASA Ames Research Center took the initiative to revisit the planar arm concept and modified a bottom entry HUT mock-up to incorporate planar arm openings. The evaluation suggested that a Shuttle-type, bottom entry HUT with planar arm openings could have acceptable don/doff characteristics. In early 1991, Hamilton Standard and ILC Dover began an internal development effort building upon NASA's work. Based on 12 years of program operational experience, an improved understanding of anthropometry, and the results of NASA's development efforts on the MKIII and AX-5 advanced space suits, it was felt that the potential benefits of system safety and reduced component cost would justify the risk associated with reevaluating a planar configuration. Today, NASA/JSC, Hamilton Standard and ILC are developing the planar arm design for eventual implementation into the NASA's EMU flight program.This paper, which describes the completed and planned activity associated with the planar HUT program, begins by describing the methods employed by Hamilton Standard and ILC to investigate the following variables: shoulder diameter, arm opening placement (relative to the constraints defined by the helmet neck ring), Display and Control Module (DCM) and Primary Life Support System (PLSS) relative to the different HUT sizes (from small to extra-large). The paper continues by describing the additional participation of several NASA/JSC personnel (including astronaut crewmembers) in a coordinated effort to evaluate the don/doff and mobility characteristics of the new HUT. Conclusions are presented along with future plans for completing the development and incorporating the changes into flight hardware.