This article explores the issues associated with developing ideas of informal inference and introduces the software package, TinkerPlots, as a tool to facilitate this development. The activities suggested in this article are intended for use with middle and secondary students (grades 6 to 10). The data and suggestions presented have arisen mainly from workshops with inservice middle school teachers and preservice primary teachers, and hence may provide models for similar sessions, as well as for activities in the classroom. (Contains 1 table and 11 figures.)