A defendant may assert a defense of impossibility. He may, for instance, argue that the target offense could not possibly have occurred, no matter how hard the defendant worked to commit it. If he argues that the impossibility was based on the fact that what the defendant was about to commit was not in fact a crime, he is asserting the defense of legal impossibility. At common law, and today, legal impossibility is a complete defense to both conspiracy and attempt. For example, if you and your friend make an agreement to have sex with a 19-year-old woman, neither of you can be found guilty of statutory rape, because having sex with a 19-year-old woman is not a crime.

However, a defense of factual impossibility will not be successful under traditional common law rules, nor in today's jurisdictions. The defense of factual impossibility is asserted when the defendant argues that, after agreeing to commit, or attempting to commit the target offense, it was impossible for him to complete the crime because of facts unknown to him at the time of the agreement, or at the time he attempted the crime. The reason this is no defense is that, had the facts been as the defendant believed them to be, the target crime would have actually occurred. So, factual impossibility is never a defense to conspiracy or attempt.

Another defense that can be asserted is withdrawal. A defendant may assert that he withdrew from the conspiracy, and thus should not be found guilty of conspiracy. He can assert a similar argument against a charge of attempt. In jurisdictions that follow the common law, withdrawal is no defense to criminal conspiracy or criminal attempt. As an example, suppose you conspired to commit larceny in the victim's hotel room. Just before entering the hotel lobby, you got cold feet and communicated your desire to withdraw from the planned crime. At common law, even if you turned and ran away, meaning you actually withdrew and the co-conspirators went ahead with the crime, you will still be found liable for both the conspiracy (to commit larceny) and for the completed crime of larceny, because of your earlier agreement.

There are some jurisdictions that today recognize a withdrawal defense under the Model Penal Code. These jurisdictions hold that a complete, effective withdrawal from a planned crime can limit a defendant's liability to the point where he effectively withdrew. The defendant, although guilty of the conspiracy, will not be found liable for subsequent crimes. For example, a defendant who communicates his withdrawal from a conspiracy to commit arson to the other conspirators, and even works to hinder the occurrence of the target crime, such as by notifying the police and fire station, will not be found liable in these jurisdictions if the conspiracy results in the intended arson.