This study tests grammatical aspect in adult Heritage Speakers (HSs) of Greek in Germany (HSs-Germany) and the US (HSs-US), a topic which has not been investigated before for this language, exploring the role of the dominant language and the default value as an acquisition strategy. In an oral elicitation task (Experiment 1) targeting the production of aspectual marking in Greek, Greek monolinguals (MSs) and HSs-Germany exhibited ceiling performance, while HSs-US were significantly less accurate. Education in Greek reliably predicted their accuracy. In a speeded Grammaticality Judgment task (Experiment 2) targeting the comprehension of aspect in a Grammaticality x Aspect repeated measures design, similar results were obtained for the grammatical conditions as in Experiment 1. In ungrammatical conditions, accuracy on aspect was affected for all groups, and this was more evident for HSs. HSs-US were overall less accurate with the morphologically marked form (perfective). Decision Times (DTs) revealed that only MSs and HSs-Germany were sensitive to aspect violations exhibiting longer DTs. Education in Greek reliably predicted accuracy and DTs. The results are discussed within the realm of heritage languages, language contact, and aspect acquisition in Greek bilingual populations. Finally, certain novel verbal forms produced by HSs are also discussed.