This study focuses on how prosodically and morphologically based OT constraints are implemented in voicing and place assimilations in English. It is revealed that prosodically based ID-ONS(V) applies to both assimilations except for assimilations occurring with irregular inflection morphemes. Morphologically based meta-constraint ranking, however, plays a role only in progressive voicing assimilation with regular inflection morphemes and regressive place assimilation with complex words. Thus, prosodically and morphologically based constraints are differently implemented in assimilation processes in English. The study shows that general faithfulness constraints should be decomposed into more specified constraints. It is also revealed that the general direction of assimilation is regressive in English and it might be reversed if morphological facts are involved in the process (Lombardi, 2001).