The document discusses 5 of the most costly network security threats faced by enterprises: 1) botnets, 2) phishing, 3) malware, 4) distributed denial of service (DDoS) attacks, and 5) increasingly sophisticated attacks. It recommends implementing key layers of control through network perimeter protections, cloud-based security services, mobile device security, and partnering with a managed security provider to help prevent threats and do more with less.