Practical recommendations are given for using popular debuggers such as NuMega Softlce, Microsoft Visual Studio Debugger and Microsoft Kernel Debugger. It is shown how debuggers work and how to resist program disassembly. The main security mechanisms of commercial programs are described, as well as methods for restoring and changing the program algorithm without source code. Much attention is paid to embedding and removing code from PE files. The material is accompanied by practical examples. The CD contains the source texts of the listings and useful utilities.