In an increasingly networked world, cybersecurity and cyber resilience are more necessary than ever to protect and secure critical cyber systems. However, cybersecurity has traditionally relied on identifying potential attacks and developing security controls to protect against such attacks, known as static security controls. These controls are insufficient for protecting against unforeseen attacks and unknown vulnerabilities including undisclosed software vulnerabilities and social engineering attacks that can circumvent static controls. Therefore, this chapter describes ongoing research in the field of active defense techniques that are more responsive to threats and can not only increase cybersecurity, but also cyber resilience. We review the state-of-the-art in techniques for active defense, categorizing approaches as either a hybrid of reactive and proactive techniques, or as simply proactive techniques. Some of these approaches are at or near technological maturity, while others require further research and development. Such approaches should be adopted more as organizations seek to not only protect against known cyber-attacks, but also to better respond to unknown threats with sufficient resilience.