This Practice Note provides practical guidance on the use of damages experts in international commercial and investment arbitration. This Practice Note considers: when a damages expert (witness) is necessary; the pros and cons (or advantages and disadvantages) of using damages (or quantum) experts; the best way to bring an expert on board; identifying the right expert; and, tips for working with experts. This topic may be referred to as: the function and role of damages experts; the role of experts in assessing damages (or quantum); a guide to damages in international arbitration; an expert’s perspective on damages; and, calculating damages in international arbitration.