Optimizing compilers can make life both easier and more complex for programmers than can their non-optimizing cousins. Easier because they can automate code optimizations that would be time-consuming, error-prone or both to do by hand. Complex because their often non-trivial code transformations can make it difficult to understand the interaction between source code, compiler and target code. This presentation will give some insights into the workings of modern optimizing compilers, and how they can help developers make programs execute faster. The presentation is part of the ongoing performance engineering course here at DIKU, but anyone interested is welcome to attend.