For most parallel and high performance systems, tuning guides provide the users with advices to optimize the execution time of their programs. Execution time may be very sensitive to small program changes. Such modi cations may be local (on loop) or global (data structures and layout). In this paper, we propose to help end-users with the tuning process through an interactivetool complementary to existing compilers and automatic parallelizers. Our goal is to provide a live tuning guide capable of detecting optimization opportunities that are not caugh tby existing tools. Our rst prototype, called caht (Computer Aided Hand Tuning), targets SMP architectures for OpenMP programs. caht relies on a v ery general technique, case based reasoning. This technique is adequate to experiment and build an easily expandable and exible system. Our rst implementation applies to scienti c codes written in Fortran 77.