This chapter explains computer security through the data science lens. It presents the basic knowledge needed by a reader who wants to secure his/her own computer. The goal of confidentiality is a very important one, and many of the solutions that have been developed for this goal are useful in other security goals as well. The goal of message integrity can be achieved by a message authentication code or a cryptographic hash function. Power failures, disk crashes, and network outages are some of the disruptive faults that can bring a system to its knees. Authentication is the goal of proving one’s identity to a machine or another human individual or organization. One of the biggest security challenges for computer users is choosing and remembering strong passwords. Malware refers to bad software and includes: viruses, worms, trojans, logic bombs, keyloggers, rootkits, backdoors, ransomware. Computations on physical devices can leak information through what are called side-channels.