The police use “entrapment” to detect and apprehend one whom they suspect is about to commit a crime by providing him with, or urging him to take advantage of, an opportunity to do so. The practice is much commoner in England than was once thought; it raises a number of relatively undiscussed problems which fall into two groups, one centring on the trapper, the other on the entrapped accused. The first group comprises such issues as: Is the trapper criminally responsible? Does the rule relating to the corroboration of accomplice evidence apply to him? If not, should there be any warning about his testimony? Does his conduct sometimes require the exclusion of his testimony as being unfairly obtained? Secondly, has the accused a defence? Has he grounds for pleading in mitigation of sentence? Before looking at these issues, we shall examine the purposes and dangers of entrapment, and we shall see how the practice sometimes prevents either trapper or accused being criminally responsible for certain offences.