Methods for dynamically instrumenting a program while the program is executing are described. In some embodiments, profiling hooks may be selectively inserted into and removed from a program while the program is running. The hooks may gather profiling information, such as the frequency and duration of function calls, for a selected set of functions. The hooks may be inserted into the program without requiring a special build or modifications to the binary by modifying machine-level instructions for the program stored in system memory. The ability to selectively insert instrumentation into the machine-level instructions stored in the system memory allows a set of functions to be selected during execution of the program and hooks for each function of the set of functions to be dynamically inserted or removed during execution of the program to precisely capture profiling information for the set of functions.