This article develops a minimal dynamic model for the acquisition and loss of network contacts in ego networks. The model shows a simple relationship between the rate at which contacts are gained, the rate of loss, and the number of contacts observed in cross-section. The model produces a novel interpretation for the characteristics of tie strength in ego networks. Some conclusions based on the analysis of the model are (a) any theory of ego networks must have a model of both the creation and destruction of ties, (b) the dynamics of tie strength are underspecified at present, (c) current methods of gathering data on ego networks provide virtually no information on weak ties, and (d) specifying the absence of ties is both more important and more problematic than previously recognized. The article concludes with some applications of the model of relevance to work on social capital.