Mental toughness (MT) is a key psychological variable related to achievement in performance domains and perseverance in challenging circumstances. We sought to understand the lived experiences of mentally tough high-altitude mountaineers, focusing primarily upon decisions to persevere or abort summit attempts. Phenomenological interviews were conducted with 14 mountaineers including guides, expedition leaders, and doctors (Mage = 44 years). A content analysis was employed to identify key themes in the data. Participants emphasized the importance of MT in extreme environments and described (...) rational, flexible, and vigilant decision-making. Turning around without summiting was the toughest decision reported, with recognition of the thin line between persevering and overstretching. In contrast to much MT literature, mountaineers accepted limits, demonstrated restraint, and sacrificed personal goals to aid others. Costly perseverance was also reported with some mountaineers described as “too tough”: overcompetitive, goal-obsessed, and biased decision-makers. These findings revealed both benefits and dangers of MT in mountaineering. (shrink)