It is a well-established principle that litigants – both plaintiffs and defendants – may not relitigate issues that have been previously adjudicated upon. This principle was recently considered by the Court of Appeal in Penner v. Niagara (Police Services Board), [2010] O.J. No. 4046 (C.A.), in which the Court affirmed the decision to strike parts of the action which had been previously adjudicated upon in a police discipline proceeding. The facts of Penner are somewhat unique and lent themselves to the application of issue estoppel.