The purpose of this article is to review the issue of the set-off defence in arbitration. While it is generally accepted that the defence of set-off is not intended to be a means of asserting a disputed claim, in practice it is often used for that purpose. Consequently, the claim presented for set-off must be covered by the arbitration clause in order for the tribunal to have jurisdiction to examine the set-off defence. Otherwise, an award made in such a case can be set aside as having been made on a matter not covered by the arbitration agreement.