This study reports results from four experiments investigating the perfective past tense of Greek in adult second language (L2) learners. The data come from L2 learners of Greek with intermediate to advanced L2 proficiency and different native language (L1) backgrounds, and L1 speakers of Greek. All participants were tested in both oral and written elicited production and acceptability judgment tasks on both existing and novel verb stimuli. The results showed that the L2 learners did not achieve native-like performance on the perfective past tense in Greek, even at an advanced level of proficiency. The L2 learners often chose verb forms that did not encode the perfective past tense. Differences to native speakers were found particularly for non-sigmatic verb forms, which contain morphological irregularities in the target language. The results of the four experiments will be discussed in the light of previous findings and accounts of inflectional morphology in adult L2 learners. Taken together, the results suggest that L2 learners rely more on stored inflected word forms and on associative generalizations than native speakers. (Contains 5 tables.)