Supplemental Instruction (SI) is a programme that is attached to difficult courses with the objective of increasing student performance and retention. However, an SI programme also has the potential to increase overall student performance and retention during the first critical year if applied to introductory courses. In this study the latter objective is investigated in an engineering educational environment. The study shows that an SI programme attached to difficult first semester courses for new engineering students has substantial positive effects on both first-year student performance and retention. Both male and female students appear to benefit from attending SI to the same extent. Some potential reasons for these improved first-year student performances are that attendance at SI sessions appears to lead to improved self-confidence, a broader network of study partners, improved study strategies and problem-solving skills and an increased ability to critically review material and work with others.