Social media have opened up new sources of information on crime to citizens which facilitates their participation in crime fighting. The labels of “do-it-yourself (DIY) policing” and “digital vigilantism” (or “digilantism”) suggest that a normative line can be drawn between forms of participation that are accepted and unaccepted by law enforcement. This chapter studies how law enforcement authorities set boundaries on what digital contributions of citizens to public security are acceptable. This law enforcement perspective is largely missing in current research into online engagement with public security. Based on a series of round table discussions with approximately 150 European law enforcement practitioners, we were able to study how they discursively define the boundaries between acceptable and unacceptable forms of public engagement in law enforcement. Citizens are considered as aides to police efforts when they engage with cases relating to their local context and when they closely collaborate with law enforcement. When online engagement goes beyond this familiar model of co-production, citizens are more likely to be considered adversaries. This concerns involvement in cases outside of citizens’ own local contexts and when they do not collaborate with law enforcement or only in a later stage in order to be able to claim their own successes. This suggests that discursive boundaries are drawn based on the resemblance with the existing operating paradigm of “community policing”.