This chapter examines the productivity of the English suffix -ize and its understanding by native and non-native English speakers. Derived from French -iser and ultimately from Classical Greek -ίζω via Latin -izāre, the suffix entered English in the thirteenth century and has since increased in productivity. Most of -ize verbs are based on Latinate words, with several ones being derived from inherited English words. I first review the historical development of -ize and its characteristics compared to other suffixes, -ify and -ate. Then, I examine the results of a survey conducted with 46 native and 48 non-native speakers to assess their understanding of -ize’s productivity. Participants rated 20 words ending in -ize: six existing words, seven non-existing words from Latinate nominals, and seven non-existing words from inherited English roots. The results show that native speakers’ understanding of the productivity of -ize is more accurate than non-native speakers’, which can be attributed to the role of school education. The findings suggest that teaching non-native speakers about derivational suffixes could enhance their understanding of English word formation.