This article deals with work on marginal stimuli (subliminal, incidental, or partially seen) as a means of experimentally studying thought processes outside awareness. There are a number of ways of introducing marginal stimuli—brief, dim, obscured, incidental to other activity—which are reviewed. The connecting links between these stimuli and their effects on the one hand and psychoanalytic theory on the other is developed. The day residues in dreams provide a useful parallel. Those stimuli that are retained are felt to make some contact with drive activity, in the way that dream activity is organized around drives. However, in addition to the importance of primary process to explain the phenomena involved, the author cites the importance of the subjects' expectations, thinking