<![CDATA[The problems in this research are (1) the expository method used in learning processes becauses lack of motivation and activeness in students, (2) the students' understanding of mathematical concepts are low. This research is an R&D type research that aims to develop and aplly a valid and effectivekotakkatik media. Kotak-katik media uses the contextual approach with ADDIE development method and the TGT cooperative learning model. The population in this research consists of 8th grade students in JHS of Muhammadiyah 1 Kudus. The samples are obtained using purposive sampling. 85.5% validity rate is obtained, meaning that this media is very valid. Effectivity analysis shows that complete individual understanding of the concepts reaches the value of87.2667 and classical completion reaches the proportion of 86.6667%.Motivation and activeness are ability in understanding the mathematicalconcepts with effect percentage of 89,9%. Application of this kotak-katiklearning media creates difference in the measure of mathematical concept understanding, in the experimental class is 87,2667 better than control class is 79.1333. Based on those results, therefore the development of kotak-katik media through contextual approach to mathematical concept understanding of straight-sided solid shapes is valid, effective, and suitable to use as a new variation in learning. Keywords: Kotak-katik, Kotak-katik Learning Media, Contextual Approach, Mathematical Concept Understanding]]>
