This chapter describes security threats that systems face when they are connected to the Internet. The authors discuss their security requirements, potential security threats and different mechanisms to combat these. In addition, the text presents the two most popular protocols (SSL and its successor TLS) to secure data transmitted over the Internet. Finally, the authors describe well known applications such as secure shell (ssh) and secure file transfer protocol (sftp) that provide a reasonable level of security for common tasks. They may be utilized as underlying building blocks to create secure, Internet enabled applications.