This article presents an ethnographic analysis of an often-overlooked element of social capital—commitment—on Coming up Roses, an online pregnancy and mothering support group. Specifically, it examines the architecture of commitment: the ways in which the site’s social and technical design influences the commitment that members feel toward the site and one another. In so doing, this account illuminates how social life and technical life mutually construct one another in online support environments. It also demonstrates that a site’s social and technical features can produce multiple and even contradictory effects, depending on the social and historical contexts in which users engage it.