A newly installed Operating system is inherently insecure. It has a certain number of vulnerabilities, depending on such things such as the age of the O.S., the amount of services it provides, the existence of initial backdoors not already patched, and the use of default policies designed without security in mind By Hardening Operating Systems we refer to the act of configuring an operating system with the aim of making it as secure as possible, so thet we minimize the risk of getting it compromised. This usually implies applying patches, changing default O.S. policies, and removing (or disabling) non-essential aplications and/or services. In this course we'll try to identify common O.S. vulnerabilities and how to defend the O.S. against them. Both UNIX (linux) and Windows type O.S. will be considered.