Developing advanced technology through the prototype phase on a system as complex as a Portable Life Support Subsystem (PLSS) for an Extravehicular Mobility Unit (EMU) is a time and resource consuming process. Experience has shown that most of the decisions controlling the life cycle cost of a system intended for operational use are made very early in the design process. By the preliminary design review most of the design-controlled cost drivers are locked into the design. To ensure a reasonable chance for the design to successfully meet mission requirements, it is best to choose the most promising, most likely-to-succeed technology available in the early stages of breadboard and preprototype development. For this reason the Extravehicular Activity (EVA) and Spacesuit Systems Branch of the Crew and Thermal Systems Division of NASA's Johnson Space Center (JSC) uses a requirements-driven, methodical, unbiased, trade study approach for the selection of a schematic arrangement of an advanced PLSS and for the subsystem technologies to be used in that schematic. Selection of advanced technologies for an evolutionary space station or a planetary (Lunar or Martian) EMU will be strongly driven by factors such as the system volume and weight, life cycle costs, reliability, safety, maintainability, etc. Use of these factors for design selection must consider their relative importance as related to mission objectives.This paper presents the general philosophy and design method which is used by the EVA and Spacesuit Systems Branch to perform advanced PLSS technology trade studies. These studies aid in the selection of an advanced PLSS schematic and illustrate to outside organizations which elements of design we consider important and why. This paper will also illustrate the importance of accomplishing a disciplined, requirements-driven, methodical, well documented approach, which is important to making future NASA decisions that do not limit creativity during the early part of the design process when it is most needed.