This article presents the application of the interactive dynamic literacy (IDL) model (Kim, 2020a) toward understanding difficulties in learning to read and write. According to the IDL model, reading and writing are part of communicative acts that draw on largely shared processes and skills as well as unique processes and skills. As such, reading and writing are dissociable but interdependent systems that have hierarchical, interactive, and dynamic relations. These key tenets of the IDL model are applied to the disruption of reading and writing development to explain co-occurrence of reading--writing difficulties using a single framework. The following hypotheses are presented: (a) co-occurrence between word reading and spelling and handwriting difficulties; (b) co-occurrence of dyslexia with written composition difficulties; (c) co-occurrence between reading comprehension and written composition difficulties; (d) co-occurrence of language difficulties with reading difficulties and writing difficulties; (e) co-occurrence of reading, writing, and language difficulties with weak domain-general skills or executive functions such as working memory and attentional control (including attention-deficit hyperactivity disorder [ADHD]); and (f) multiple pathways for reading and writing difficulties. Implications are discussed. [This is the online version of an article published in "Journal of Learning Disabilities."]