Surreptitious attacks are usually performed by skilled intruders who breach locks so that illegal entry is not readily apparent and investigations are often delayed. Four surreptitious approaches are reviewed -- use of illicit keys, circumvention of internal barriers, manipulations of internal barriers, and shimming. Commonsense ways to minimize lost, stolen, or duplicated keys are reviewed. Lock manipulation techniques such as picking are described. Impressioning and decoding, two methods of making a key for a particular lock, and the locks most vulnerable and resistant to these techniques are described. The large majority of lock attacks involve the use of force by intruders without the skills to perform surreptitious attacks. Any lock is vulnerable to force given enough time, and illegal entry is readily apparent. Most forceful attacks are directed at bolts and cylinders. Basic techniques for attacking bolts, such as prying, punching, and sawing are reviewed, along with which locks are the least and most vulnerable to these techniques. Cylinder attacks by prying, punching, drilling, pulling, and wrenching are also reviewed. The article emphasizes that any lock can be defeated with sufficient time and expertise, but that an awareness of specific threats to locks can help security managers choose the best locks. 18 exhibits.