Sponsorship: Los Alamos National Labs (LA-UR-03-7525). Field programmable gate arrays (FPGAs) are sensitive to radiation-induced single event upsets (SEUs) within the configuration memory. Triple modular redundancy (TMR) is a technique commonly used to mitigate against design failures caused by SEUs. This paper evaluates the effectiveness and cost of TMR on two different counter designs in the presence of SEUs. The evaluation measures the reliability, area cost, and speed of different TMR styles. The tests show that when feedback TMR is used with triplicated clocks, it is possible to have counter design which is insensitive to any single configuration upset.