Summary: One of the key assumptions of conventional line transect (LT) theory is that all animals in the observer's path are detected. When this assumption fails, simultaneous survey by two independent observers can be used to estimate detection probabilities and abundance. Models are developed for such surveys for both grouped and ungrouped perpendicular distance data. The models unify and generalize existing line transect and mark-recapture models. They provide a general framework for the estimation of abundance from LT surveys in which detection of animals on the trackline is not certain and/or the probability of detection depends on perpendicular distance and additional covariates. Existing LT models in the literature are obtained as special cases of the general models. We use data from a shipboard line transect survey of Antarctic minke whales to illustrate use of the models.