Protecting associational freedom is a core, independent yet unappreciated part of the Fourth Amendment. New surveillance techniques threaten that freedom. Surveillance is no longer forward looking. Law enforcement can obtain the same, if not more, information about all of us by looking backward. Forward-looking surveillance has limits. Some limits are practical such as the cost to place a person in a car to follow a suspect. There are also procedural limits, such as the requirement that surveillance relate to criminal activity. In addition, surveillance such as wiretapping and using a GPS tracker often requires a warrant. Warrants involve review by a neutral magistrate. The warrant sets limits on what information may be collected, how it is collected, and how it can be used. The surveillance is also time limited and requires continual justification to a judge, or the surveillance will be shut down. With backward-looking surveillance all of these protections are gone. Law enforcement can now use low-cost technology to track us or need only ask a business for the record of where we went, whom we called, what we read, and more. Revelation of the NSA's vast Prism surveillance project is but the most recent example of law enforcement engaging in this sort of over-reaching surveillance. The FBI has previously deployed similar programs to read mail, obtain lists of books read, demand member lists, and generate watch lists of people to round up in case of national emergency. The efforts vary; the harm is the same. Law enforcement has a perfect picture of our activities and associations regardless of whether they are criminal. With digital records these harms are more acute. Once the data about our activities is gathered, law enforcement may keep that data indefinitely. They have a data hoard. That hoard grows with each new data request. Once created, the hoard can be continually rifled to investigate us but without any oversight. In short, data hoards present new ways to harm associational freedom. Yet, our current understanding of associational freedom is thin. We over-focus on speech and miss the importance of the precursors to speech -- the ability to share, explore, accept, and reject ideas and then choose whether to speak. Recent work has shown, however, that the Constitution protects many activities that are not speech, for example petition and assembly, because the activities enable self-governance and foster the potential for speech. That work has looked to the First Amendment. I show that these concerns also appear in Fourth Amendment jurisprudence and work to protect us from surveillance regardless of whether the acts are speech or whether they are private. Drawing on Fourth Amendment principles, I show how warrant procedures, especially the idea of return, which would require deletion of data after an investigation, must be in place for backward-looking surveillance. This shift will allow law enforcement to access data, but limit the ability to overreach and threaten associational freedom. In short, when new surveillance techniques threaten associational freedom, they must be subject to proper Constitutional limits. This Article explains why those limits are needed, when they must be in place, and how they operate.