It is a fundamental principle of the Rules of Civil Procedure in Ontario, that parties involved in civil litigation have a duty to disclose all relevant information to one another. This should occur well in advance of any trial taking place. The intent is to avoid any surprises at trial, pin down admissions of fact, and promote earlier settlement of cases.

Court rules therefore provide that parties must agree to, and update on an ongoing basis, a written discovery plan within 60 days of delivery of the reply and defence to counterclaim, if any. The discovery plan sets out the scope of documentary discovery, the names of the people who will be examined and the timelines for completing oral and documentary discovery.

Once the discovery plan is completed, each party is obligated to serve an affidavit of documents on the other party within the timetable established in the plan. An affidavit of documents is supposed to list all documents that are, or were, in the party's possession and that are relevant to any matter in issue in the action. Each party is entitled to examine and receive copies of such documents from the other, except for privileged communications between lawyer and client.

The obligation to disclose relevant documents is ongoing throughout the litigation process. If, after preparing an affidavit of documents, a party comes into possession of a document that is relevant to a matter in issue in the lawsuit or otherwise discovers that the previous affidavit of documents is inaccurate or incomplete, the party must prepare a supplementary affidavit of documents. There are various sanctions and remedies for failure to comply with this requirement of disclosure, but they have to be accessed through a motion made in court.

After all the documents have been exchanged in an action, the parties are entitled to examine each other, under oath, to 'discover' all the evidence the other party intends to rely upon in support of its case. There are time limits as to how much discovery questioning is permitted.

The parties are examined by the opposing party's lawyer in an office setting, in the presence of a court reporter. Their own lawyer sits beside them to monitor the questioning and assist the witness. Questions and answers are recorded and a transcript may later be prepared, which can be used at motions and at trial for certain purposes. When a party being examined during an examination for discovery does not have the information needed to answer a question at that moment or does not have a document that is requested, the party being examined can agree to provide the answer to the question, or the relevant document, after the examination is completed (this is called an "undertaking").

If the person being examined does not provide the answer to the undertaking within a reasonable period of time, the lawyer for the other party can bring a motion to a judge requesting the information. If such an order is made, the party who failed to fulfill their undertaking within a reasonable time may be ordered to pay the costs of the other side for having to bring a motion to compel an answer to the undertaking.