This research is concerned with frequency locking phenomena in spatially extended media and addresses the effects of pattern formation on resonance behavior. The study has been motivated by recent experiments on temporally driven Belousov-Zhabotinsky reaction-diffusion systems focusing on standing-wave patterns. We study pattern formation mechanisms and parameters ranges where resonant and non-resonant standing-wave patterns are developed. The analysis is based on the complex forced Ginzburg-Landau equation which describes universal dynamical behavior of periodically driven oscillatory media. Among our results we show that in extended systems spatial structures and instabilities may reduce or extend the boundaries of frequency locking so that the resonance ranges for a single oscillator do not always coincide with resonance ranges in extended systems. In addition, we explain experimental results obtained on the Belousov-Zhabotinsky chemical reaction and extend the concept of frequency locking to spatially extended systems.