The purpose of this mixed-method study is to determine the effect of integrating Engineering design on high school students' scientific reasoning and science achievement. Thirteen eleventh-grade students participated in this eight-week long study. The intervention engaged students in the Engineering design of a functional Dialysis model. Students followed eight engineering practices to design and build their models. The researcher-developed Dialysis Reasoning test assessed students' reasoning in six areas: inductive, deductive, correlational, causal, hypothetical-deductive, and analogical reasoning. Students' scores on the reasoning and achievement tests demonstrated significant improvement in comparison to the control group. Students' Dialysis sketches revealed the progress in their conceptual understanding and proposed four levels for analyzing the drawings. This study suggests the importance of scaffolding in the design activity. Engineering design can enhance students' achievement and scientific reasoning however, some students fail to reason when they design. Providing students with sustained help and assistance is essential to establish relevant connections between design and scientific reasoning. This mentoring would require careful planning and depends on giving students frequent opportunities to critically reflect on their design and connect it to the related scientific theories and principles. [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.]