In this chapter we will focus on examining computer network traffic and data. A computer network combines a set of computers and physically and logically connects them together to exchange information. Network traffic acquired from a network system provides information on data communications within the network and between networks or individual computers. The most common data types are log data, such as Kerberos logs, transmission control protocol/Internet protocol (TCP/IP) logs, Central processing unit (CPU) usage data, event logs, user command data, Internet visit data, operating system audit trail data, intrusion detection and prevention service (IDS/IPS) logs, Netflow1 data, and the simple network management protocol (SNMP) reporting data. Such information is unique and valuable for network security, specifically for intrusion detection and prevention. Although we have already presented some essential challenges in collecting such data in Chapter I, we will discuss traffic data, as well as other related data, in greater detail in this chapter. Specifically, we will describe system-specific and user-specific data types in Sections System- Specific Data and User-Specific Data, respectively, and provide detailed information on publicly available data in Section Publicly Available Data.