The proposed study involves a single-blind, Phase I randomized controlled trial (RCT) on the effects of supervised treadmill walking exercise training on learning and memory, hippocampal structure/function, and cardiorespiratory fitness outcomes in fully-ambulatory (i.e., EDSS ≤ 4.0) persons with multiple sclerosis (MS) who have impairment in learning new information. This study will compare the effects of 3 months of supervised, progressive (both intensity and duration) chronic treadmill walking exercise (designed based on pilot work [Sandroff et al., 2015; 2016a; 2016b] and American College of Sports Medicine (ACSM) guidelines) with those of an active control condition (i.e., stretching-and-toning activities) on learning and memory, hippocampal volume and resting-state functional connectivity, and cardiorespiratory fitness. Learning and memory represents the primary outcome, whereas hippocampal volume and resting-state functional connectivity and cardiorespiratory fitness represent the secondary outcomes. Such an investigation will provide the first Class I evidence for the effects of treadmill walking exercise training on those outcomes in MS.