Externalism about knowledge is thriving in contemporary epistemology. Nonetheless, externalism is too often caricatured as merely reliabilism, too often reduced to simply externalism about justification, and rarely considered as a cohesive family of related but importantly different views. Externalism About Knowledge addresses all of these issues by bringing new essays from leading externalist epistemologists working on seven different branches of this tradition: process reliabilism, tracking views, safety views, virtue epistemology, proper functionalism, naturalized epistemology, and knowledge first epistemology. This collection highlights their unity, their differences, their interconnections, and their most recent challenges, developments, and extensions.