Shared resources, such as the Internet, have created a highly interconnected cyber-infrastructure. Critical infrastructures in domains such as medical, power, telecommunications, and finance are highly dependent on information systems. These two factors have exposed our critical infrastructures to malicious attacks and accidental failures. Many malicious attacks are achieved by malicious code or malware, such as viruses and worms. Given the deleterious affects of malware on our cyber infrastructure, identifying malicious programs is an important goal. Unfortunately, malware detectors have not kept pace with the evasion techniques commonly used by hackers, i.e., the good guys are falling behind in the arms race.