Developing correct, fast C or C++ GNU/Linux programs requires more than just understanding the GNU/Linux operating system and its system calls. In this appendix, we discuss development tools to find runtime errors such as illegal use of dynamically allocated memory and to determine which parts of a program are taking most of the execution time. Analyzing a program's source code can reveal some of this information; by using these runtime tools and actually executing the program, you can find out much more.