Comparing the Effectiveness and Efficiency of Two Methods of Teaching Geometric Shape Concepts to Students with Mental Retardation Rüya Güzel ÖZMEN*, Hilmi ÜNAL** Abstract The purpose of this study was to compare the effectiveness and efficiency of two teaching models, the expository presentation of Merrill and Tennyson’s model and Gagne’s model, in acquisition and maintenance of square and triangle concepts in students with mental retardation. The participants of the study were chosen from students with mental retardation who attended a private special education center located in Kahramanmaras. Three boys and one girl participated in the research. The design of the study was an adapted alternating treatment design. Two criterion referenced tests were developed and administrated to assess the discrimination levels of square and triangle in participating students. The results of the study showed that the Gagne model was more effective than the Merrill...