This chapter examines some aspects of the influence of the Sorites Paradox in psychology. The first section 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 categorisation for items that differ very little. The second section emphasises the centrality of probabilistic modelling to represent the way in which small differences between stimuli affect decisions of membership under a common category. The third section focuses on experimental data concerning unordered transitions between prototypes, then the final section looks at data concerning ordered transitions between prototypes (dynamic Sorites).