This chapter explores classic security attacks on hardware systems, focusing on vulnerabilities stemming from common practices in hardware design and manufacturing. It delves into various attack vectors, including hardware Trojans, IP piracy, side-channel attacks, and counterfeiting. By examining both invasive and noninvasive attacks, the chapter highlights the growing need for robust detection and mitigation techniques. It also emphasizes the intricate relationship between software and hardware vulnerabilities, demonstrating how software-based attacks can exploit hardware weaknesses and vice versa.