Professional liability insurance (PLI), also called professional indemnity insurance (PII) and commonly known as errors & omissions (E&O) in the US, is a form of liability insurance which helps protect professional advising, consulting, and service-providing individuals and companies from bearing the full cost of defending against a negligence claim made by a client in a civil lawsuit. The coverage focuses on alleged failure to perform on the part of, financial loss caused by, and error or omission in the service or product sold by the policyholder. These are causes for legal action that would not be covered by a more general liability insurance policy which addresses more direct forms of harm. Professional liability insurance may take on different forms and names depending on the profession, especially medical and legal, and is sometimes required under contract by other businesses that are the beneficiaries of the advice or service.

Coverage almost always provides for the defense costs, including when legal action turns out to be groundless. Coverage does not include criminal prosecution, nor a wide range of potential liabilities under civil law that are not enumerated in the policy, but which may be subject to other forms of insurance. Professional liability insurance is required by law in some areas for certain kinds of professional practice.

The primary reason for professional liability coverage is that a typical general liability insurance policy will respond only to a bodily injury, property damage, personal injury or advertising injury claim. Other forms of insurance cover employers, public and product liability. However, various professional services and products can give rise to legal claims without causing any of the specific types of harm covered by such policies. Common claims that professional liability insurance covers are negligence, misrepresentation, violation of good faith, and inaccurate advice. Examples:

Professional liability insurance policies are generally set up based on a claims-made and reported basis, meaning that the policy covers only those claims made and reported to their carrier during the policy period. More specifically a typical policy will provide indemnity to the insured against loss arising from any claim or claims made during the policy period by reason of any covered error, omission or negligent act committed in the conduct of the insured's professional business during the policy period. Claims which may relate to incidents occurring before the coverage was active may not be covered, although some policies may have a retroactive date, such that claims made during the policy period but which relate to an incident after the retroactive date (where the retroactive date is earlier than the inception date of the policy) are covered. Retroactive cover is usually offered as an additional option to include cover within a policy for work one has already done or services one has already provided.

Coverage does not include criminal prosecution, nor all forms of legal liability under civil law, only those specifically enumerated in the policy. Unlike general liability coverage, professional liability coverage extends coverage for acts that are intentional, so long as the damages caused were not intentional.

Some policies are more tightly worded than others. While a number of policy wordings are designed to satisfy a stated minimum approved wording, which makes them easier to compare, others differ dramatically in the coverage they provide. For example, a breach of duty may be included if the incident occurred and was reported by the policyholder to the insurer during the policy period. Wordings with major legal differences can be confusingly similar to non-lawyers. Coverage for "negligent act, error or omission" indemnifies the policyholder against loss/circumstances incurred only as a result of any professional error or omission, or negligent act (i.e., the modifier "negligent" does not apply to all three categories, though any non-legal reader might assume that it did). A "negligent act, negligent error or negligent omission" clause is a much more restrictive policy and would deny coverage in a lawsuit alleging a non-negligent error or omission.

Coverage is usually continued for as long as the policyholder provides covered services or products, plus the span of any applicable statute of limitations. Canceling the policy before this time would in effect make it as if the insured never had coverage for any incidents since any client could bring any case with regard to any such services or products that occurred before the statute of limitations cut-off point. A break in coverage could result in what is called a "gap in coverage", which is the loss of all prior acts.

The negligent act is called medical malpractice and the insuring contract is called malpractice insurance.