Abstract: A blocking device for a magnetic-stripe or EMV® card reader, such as readers on an Automated Teller Machine (ATM), point of sale (POS) machine, or gas station pump. The blocking device securely attaches to the transaction machine such that unauthorized removal of the blocking device is difficult and likely will attract attention. The blocking device surrounds exposed portions of a magnetic-stripe card reader housing such that a skimming mold that fits over both the housing and the blocking device will stand out and be obvious to a user of the transaction machine. Also, a skimming mold fitting over the blocking device may prevent a card holder from properly inserting a magnetic-stripe card into the card reader. The difficulty of placing a skimming mold under or over the blocking device likely will deter a potential criminal skimmer from attempting to attach a skimming device in the first place.