Malpractice is a professional’s failure to use minimally adequate levels of care, skill or diligence in the performance of the professional’s duties, causing harm to another. In New York, attorney malpractice is defined as a "deviation from good and accepted legal practice, where the client has been proximately damaged by that deviation, but for which, there would have been a different, better or more positive outcome.

Malpractice typically occurs when a professional fails to exercise his or her professional skills in an assignment at the necessary standard of care, skill and learning applied under the circumstances by the average prudent reputable member of the profession in the "community". The analysis is based upon the standard of care for the professional in the community" what other professionals in the same field do for their clients who are located in the same geographic area. In New York, courts will hold all attorneys to the same standard of professional performance.

The first necessary element is a professional relationship. In order to sue for professional malpractice, the plaintiff must have retained the attorney. There must of course be a relationship in privity, between the professional and the plaintiff such that the professional owes the plaintiff a duty. In attorney malpractice either a written retainer, proof that the attorney engaged in work or proof that the attorney appeared for the client is necessary. While in litigation often there is clear proof of representation; in transactional settings, representation may be less clear. Proof to a jury’s satisfaction of actual representation must be demonstrated. This proof may come from the correspondence of the professional, from papers authored by the attorney or from litigation documents.

The first element of a relationship between the client and the professional was previously discussed. The second element, deviation, is shown by evidence, not necessarily expert, which shows that the acts of the professional fell so below the good and accepted practice of law in New York, that a jury would be permitted to find that the acts below standard.

Expert testimony is necessary when the deviation is subtle; an example could be the failure to supply an affidavit of merits to restore a case marked off calendar, the failure to respond to a CPLR 3216 notice, or failures in response to a motion for summary judgment. Expert testimony is not always necessary however. None is needed to demonstrate the deviation in failing to file within the statute of limitations. Bad outcome do not necessarily equal a deviation. Furthermore, questions of judgment of strategic choice cannot serve as the basis of malpractice. An attorney is permitted the reasonable choice of strategy, if supported by acceptable reasoning. The strategic choice must be reasonable both objectively and subjectively. The difference between strategic choice and mistake are subtle, and create the most difficult cases.

The third element of proximate cause encompasses both the typical analysis that arises in all negligence litigation and the additional element of "but for." The plaintiff must demonstrate not only that the deviation was a substantial cause of the poor outcome, but must additionally show that "but for" the deviation there would have been a different, better or more positive outcome. An example of this potential difficulty arises in an automobile accident. No matter how many deviations are shown, it may be that the maximum insurance for the other driver limits the recovery. If that is true, it will be impossible to show that "but for" the deviation, more than the policy limit was available and could have been recovered from the defendant.