Many middle school students come to Computer Science (CS) classes without any previous CS instruction or any Computational Thinking (CT) skills that are needed to be successful. To overcome this, many curriculums and programming environments have been used to engage students and to instill a love of CS. Which curriculum should be used to develop CT skills that will also increase positive attitudes towards CS so students will continue in this field of study in higher grades? The purpose of this mixed method action research study was to evaluate how Code.org's block-based programming curriculum affects middle school CS students' CT skills, and their attitudes towards CT and CS. This study explored the following two research questions: (1) How and to what degree did Code.org's block-based programming curriculum in game design affect middle school students' CT skills? (2) How and to what degree were the differences in middle school students' attitudes towards CT and CS after participating in a unit in block-based programming? This study implemented a game design curriculum in Code.org's block-based programming environment over the course of 11 weeks. Participants included 16 eighth grade students at a middle school in South Carolina. The qualitative data collected were student interviews and the quantitative data were pre- and post-tests, surveys, and student artifacts. Qualitative data analysis included inductive thematic analysis. Quantitative data analysis included descriptive statistics and paired samples t-tests. Students' artifacts showed that students learned how to code in Code.org's block-based programming environment. There was a statistically significant increase in participants' algorithmic thinking, debugging, and pattern recognition skills. The increase in participants' abstraction skill was not statistically significantly different. Qualitative themes revealed that participants enjoyed block-based programming as evidenced by their references to the elements of game design during the interviews, but the quantitative findings revealed that students' attitudes towards CT and CS did not have any statistically significant increase. This may be due to the students' performance levels and that not all students perceive coding as easy. Future directions for research and practices are discussed. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]