The present research investigated the development of English modality in the written discourse of Arab second language (L2) English learners across six levels of English proficiency. Two hundred texts were randomly selected from each of the six levels resulting in a total of 1,200 texts. Following the concept-oriented approach (CoA) to second language acquisition (SLA), modal expressions were analyzed for frequency, type and combinations of modal auxiliaries and verbs. Results indicate that initially learners express the concept of modality with limited linguistic means at their disposal such as over reliance on the primary modals can and will. Expression of this semantic concept becomes more productive and variant as learners progress in their language proficiency. More forms and types of modal expressions emerge and learners make clear distinctions between forms and meanings.