Energy Recovery, Inc.'s Big Rotor Pressure Exchanger is the result of over 100 years of technology development. Work exchanger devices developed for seawater RO plants are considered by many to be the most important breakthrough in desalination in the last 10 years. What has made this breakthrough possible and how have these devices affected the job of designing SWRO systems? This paper presents the challenges and solutions of the design and deployment of the PX. PX performance data and engineering design considerations from operating SWRO-PX process configurations and PX arrays will be provided. Recent technical advances and future products will be discussed.