For most, the loss of a material possession is an infrequent occurrence usually prevented through a variety of vaguely noticed routines. For the sociologist, the occasion offers a natural experiment in how individuals deal with a sudden threat to the order, expectations, and material scaffolding sustaining their usual way of life. Drawing on 600 naturally-occurring cases of loss collected through observations around lost and found booths and counters, interviews with individuals recruited through lost and found websites, and guided journaling by college undergraduates about their everyday misplacements, this dissertation explores the social impact of a missing possession. Tracing losses from the moment of their discovery, I show how losing parties must carve out space from their ongoing biographies if a loss is to gain foothold as a social matter. In taking up the theme of `a loss' as a matter to resolve, individuals will act to ensure that their immediate activities will go on as planned, that they will maintain untroubled relations with others, or that they will feel "complete." Yet a central paradox is that the object's loss may disturb them even while they concede its dispensability. They find that what they have lost is not something with an undeniable sentimental meaning or something that allows for some practical ability, but rather something whose absence reveals a breakdown in the self's absorption in the world. It is not that an object has been lost, but rather they have. Until they change their measures of safekeeping, get their memory checked by a doctor, get rid of extraneous possessions, etc. they cannot let go of the loss and assume that they are properly immersed in the world. It is the loss itself, not a particular object's absence that unsettles. In other chapters, the dissertation explores losses as revelatory agents, providing the grounding on which individuals reflect on others, the place of the mishap, and the meaning of the object itself. As a lens pointed away from self, losses sometimes distil social relations down to basic concerns of trust and respect. Individuals know that what they might understand as a fluke or rare occurrence within their own biography could get misinterpreted by those not privy to a more longitudinal perspective on their life. They sense that their `rare occurrence' may, for another, complete the gestalt of a "screw-up." In orienting outward toward the world of adventitious finders, they conduct experiments on the sympathy of groups like "the French" and places like "Harvard Square," hoping that unknown others who stumble upon the lost object adopt their meaning that it is a `loss' rather than a `gain.' They learn that to be understood as competent and trustworthy by others, they must demonstrate a facility for treating certain features of the inanimate world as part of their physical body. Each loss occasions an existential reflection on ease of movement through life, not as an abstract exercise removed from ongoing life, but within the dramas they currently find themselves. This is a study of how loss events and their objects are given meaning within the situations and biographies in which they unexpectedly occur.