One of the aims of instruction is to train individuals who can solve the problem. For that reason, future teachers should have problem solving skills. Thus, the aim of this study is to examine the change over the years of problem solving skills of pre-service elementary mathematics teachers. The participants consisted of 1st, 2nd, 3rd and 4th class of pre-service teachers in Elementary Mathematics Teacher Education Program in Karadeniz Technical University. For each class, three students having low, medium and high academic level were selected. In concordance with the purpose of study, three non-routine problems were asked to pre-service teachers accompanied by clinical interviews. The research lasted for three weeks in second semester of the academic year of 2009-2010 at the Faculty of Fatih Education. In problem-solving process, problem solving strategies used by teachers and teachers' thoughts are summarized in tables. For the first question; while first-year students haven't used any strategies, other students have used making a drawing and accounting for all possibilities strategies. For the second question; while first-year students have only used making a drawing strategy, beside this strategy, others have used organizing data and adopting a different point of view strategy. For the last question; while first-year, second-year and third-year students have used making a drawing and finding a pattern strategies, second-year, third-year and senior students having high academic level have utilized organizing data as well as these strategies and answered the problem. Therefore, it is suggested that problem solving oriented lessons should be heightened so as to improve problem solving skills of pre-service elementary mathematics teachers. (C) 2011 Elsevier Ltd.