The Old Assyrian private archives excavated at Kültepe contain mainly letters and legal texts. These were originally wrapped in clay envelopes on which the sender(s), of the letters, or the witnesses and parties, for legal texts, would imprint their personal cylinder or stamp seals. These envelopes were meant to protect the letter during its transport or to give its legal value to a contract. Fragments of envelopes are numerous among the archives, and some complete envelopes with their tablet still preserved inside have also been found. However, except for their seal impressions, envelopes have often been neglected because any text on them might not be seen to be primarily important. Textual data linked to the enclosing of a tablet in an envelope, and applying a seal on it, document the circumstances and the actors involved in closing of a letter or the certification of a legal document. This study of Old Assyrian envelopes is based on the corpus excavated during the 1993 season at Kültepe, in the house of Ali-ahum and his son Aššur-taklāku. The presence of envelopes and envelope fragments in this archive will be tentatively explained. A detailed study of the materiality of the envelopes will then be made in order to understand their manufacture. Descriptions here of hands-on experiments allow a better understanding of the techniques used to wrap tablets in envelopes, and the constraints while making them.