This chapter covers Windows and Linux architecture, settings, and security. The Windows section discusses its GUI, kernel, file systems (particularly NTFS), and boot mechanism from MS-DOS to contemporary NT-based PCs. Task Manager, Resource Monitor, Event Viewer, and WMI are essential monitoring and troubleshooting tools. Students study Windows memory, threads, processes, services, and registry configurations, focusing on vulnerabilities and Local Security Policy and Windows Defender security. The chapter covers network setup tools like netstat and PowerShell. The Linux part covers its history, open-source nature, and distributions like Ubuntu and Kali Linux. It covers Linux commands, the shell interface, file system structure, user permissions, and malware detection. Linux is important in security operations centers (SOC), forensic investigation, and intrusion detection. This chapter teaches students how to safeguard and monitor operating systems and how they work.