On February 3, 2014 Piotr Suski presented a lecture on “Contractual penalty in energy sale contracts” for participants of the workshop entitled Expert of practical enforcement of the Energy Law, organized in Warsaw by MM Conferences. The lecture was devoted to the problems arising from application of contractual penalty in energy sale contracts on the grounds of civil law (especially regulations concerning consumers) and competition law. It took into account also decisions of the Office of Competition and Consumer Protection as well as the Court of Competition and Consumer Protection related to demanding of contractual penalties from contractors.