Learning whether English nouns are countable or not is a source of great difficulty for many ESL/EFL learners. In the present study, a grammaticality judgement task comprised of a range of nouns representative of the different facets of the countability system in English was distributed to 82 native speakers of English (NSs) and 98 non-native speakers (NNSs). Participant responses provided both quantitative and qualitative data for analysis. The findings indicate that there are significant differences between the judgements made by the NSs and NNSs both quantitatively and qualitatively. It is also evident from the data that the judgements made by the NNSs reveal poor awareness of countability and plural marking in English, an over-reliance on noun countability preferences (i.e. rigidly memorising whether a particular noun is countable or uncountable), and a certain illogicality in their explanations. The article culminates with a brief discussion of some pedagogical implications in relation to countability and plural marking.