What are the basic conditions (arkan) of Nikah in Islam?

A valid Nikah generally requires: the free and clear consent of both the bride and groom, an offer and acceptance (ijab and qabul) made in one sitting, two adult witnesses of sound mind, and a Mahr (dowry) specified for the bride, which becomes her exclusive property. Details such as whether a guardian's (wali's) involvement is a condition of validity, and how these elements are documented, can differ somewhat between schools of thought, so it's advisable to have the ceremony conducted or reviewed by a knowledgeable scholar.