The objectives of this study were to: (1) develop a working model based on Pierre van Hiele and Dina van Hiele-Geldof's five levels of thought development in geometry; (2) characterize the thinking in geometry of sixth (N=16) and ninth (N=16) graders in terms of these levels (examining at what level they are at, if they show potential for progress within a level or to a higher level, and what difficulties they encounter); and (3) to analyze the grades K-8 geometry strand of three commercial textbook series. A wide range in levels of thinking among the subjects was found; some were consistent in identifying, naming, comparing and operating on geometry figures; others were able to give informal deductive explanations (level 2). It was also found that the inability to advance in level of thinking may be related to their deficiencies in language, both in knowledge of geometry vocabulary and ability to use it precisely and consistently. In addition, textbook material on geometry provides students with little opportunity to make progress to higher levels of thinking and may actually impede such progress by concentrating on level 0 thinking and reducing the level of thinking for topics which can be treated at levels 1-2. (JN)