Social and moral concepts associated with the location and ownership of property are crucial factors in constructing people's perception of what constitutes an opportunity to steal. Physical protection of property provides only a secondary barrier. Recidivism rates among burglars are among the highest of all types of offenders. A study of 40 haphazardly selected persistent offenders indicates that most set out with the intent to burgle and rarely are unsuccessful in finding an opportunity to do so. The consensus among those studied is that there is little a householder could do to keep a determined burglar out. Even if crime prevention activities could direct the burglar away from houses, there are indications that they would merely move to a different line of thievery. Most of those studied had previous convictions for other kinds of property offense, such as stealing motor vehicles, shoplifting, burglary of commercial premises, and check frauds. Probably, the idea that opportunity can determine whether a burglary occurs can only be applied to some of those burglaries committed by amateur juveniles. However, there is evidence that improving the physical security will have as little impact on juvenile burglars as on adults. Investigations of supply factors (the number of opportunities available) need to be completed with studies of demand -- the outlook and values of people likely to take advantage of any opportunities that exist. Fourteen references are included.