This chapter examines some aspects of the influence of the sorites paradox in psychology. Section 1 starts out with a brief discussion of the analysis of slippery slope arguments in the psychology of reasoning, to introduce the relevance of probabilistic considerations in that domain. We then devote most of this chapter to the analysis in psychophysics and in the psychology of concepts of the complex relationship between discrimination and categorization for items that differ very little. Section 2 emphasizes the centrality of probabilistic modeling to represent the way in which small differences between stimuli affect decisions of membership under a common category. Section 3 focuses on experimental data concerning unordered transitions between prototypes, then section 4 looks at data concerning ordered transitions between prototypes (dynamic sorites).