Belgian law recognizes the concept of professional secrecy, which is a principle of legal ethics belonging to the essence of the lawyer's profession. Its protection also constitutes an essential element in the safeguarding of individual liberty in a free society and is likewise intended to ensure the proper administration of justice. Accordingly, the duty of professional secrecy is considered a matter of public order.

The duty of professional secrecy prohibits members of the bar, who are authorized to pursue their professional activities under the professional title of avocat/advocaat/Rechtsanwalt — for the purposes of this chapter referred to as "lawyers," from disclosing oral or written information related to the representation of a client.

This principle of professional secrecy is not only a lawyer's right, but also a duty. A breach of such duty is subject to sanctions under the Criminal Code, except in limited circumstances where the law requires or permits the lawyer to disclose the information. Even in cases where the law exceptionally authorizes release of the information, the lawyer may still be sanctioned by the bar authorities.

In complying with their obligation of professional secrecy, lawyers have the right and the duty to refuse to hand over documents in their possession to regulating or seizing authorities and to refuse to testify before the court both in criminal and civil cases.

Whereas in common law systems the attorney-client privilege usually attaches to confidential communications between client and lawyer that are created for the purpose of giving or receiving legal advice, the duty of professional secrecy applies to all information related to the representation of a client, whatever its source. In other words, it is not so much that the communication itself is privileged, but that the lawyer is under a duty to not disclose the information in it. This means that as a general rule, the duty of secrecy forbids lawyers from testifying to any fact that was revealed to them during the course of the exercise of their profession, unless the testimony is necessary for the representation of the client or for the lawyer's own defense in a criminal or civil case.

As a consequence, whereas the attorney-client privilege in common law systems is usually a right that belongs to the client and can be waived by the client, in Belgium professional secrecy as a general rule cannot be waived by the client, subject to certain exceptions to allow more fundamental values to be protected (e.g., the right to defend oneself or to prevent a risk to life or health). The protection cannot be invoked in order to help a client commit a crime.

It merits emphasis that professional secrecy is limited to those cases where the lawyer is acting in the capacity of a lawyer and not, for example, as a board member of a company, or as the trustee of a bankruptcy.

Finally, professional conduct rules of the Belgian bar prescribe that correspondence between Belgian lawyers is, as a general rule, also confidential and cannot be used as evidence in court proceedings. However, this confidentiality is restricted to relations between lawyers and is not related to professional secrecy, except when the information exchanged between lawyers is in itself privileged.