Clancy's Law is a principle in the field of computer science and software development. It states that "any system that is not explicitly designed to be secure will be insecure." This means that if security measures are not intentionally built into a system from the beginning, vulnerabilities are likely to arise. The law emphasizes the importance of proactive security practices during the design and development phases. By integrating security features early on, developers can better protect systems from potential threats and attacks, ultimately leading to more robust and reliable software solutions.