The purpose of this investigation is to demonstrate the safety and effectiveness of the EndoStim® Lower Esophageal Sphincter (LES) Stimulation System in the treatment of subjects with gastroesophageal reflux disease (GERD). This investigation is a multicenter, randomized, double-blind, sham-controlled clinical investigation. After the implant procedure, subjects will be randomized to either the Treatment Group (immediate stimulation) or Control Group (delayed stimulation) for a 6-month, double-blind phase followed by an additional open-label treatment phase in which all subjects will receive electrical stimulation therapy for a total of 12 months. Subjects continue on stimulation treatment and an extended open-label follow-up phase includes an 18 month post-stimulation phone interview followed by annual visits through 5 years post-stimulation.