Protecting associational freedom is a core, independent yet unappreciated part of the Fourth Amendment. New surveillance techniques threaten that freedom. Surveillance is no longer primarily forward looking. Today, changing technology allows law enforcement and intelligence services to obtain the same, if not more, information about all of us by looking backward. This shift massively expands the government’s ability to examine, investigate, and deter exercise of the freedom of association. Forward-looking surveillance has limits that don’t apply to backward-looking surveillance. Some limits are practical such as the cost to place a person in a car to follow a suspect. Some are procedural, such as the requirement that surveillance relate to ...