Numerous studies have reported that extensive reading (ER) has a positive influence on affect. Recent studies suggest that motivation for ER changes. This is in line with recent developments in second language (L2) motivation research that have highlighted the complex and dynamic nature of L2 motivation. This study presents a model of complex and dynamic motivation for ER. This qualitative study examined 9 Japanese as a foreign language learners' motivation for ER. The participants were encouraged to read as many Japanese books as possible outside class for 5 to 7 months. Data from interviews and journal entries were analyzed for factors influencing their motivation. The participants' motivation changed as different factors interacted, leading to different patterns of engagement with ER, which fit within the model. This suggests the value of using a complex and dynamic approach to L2 extensive reading motivation. Implications concern the importance of varied materials and of making ER obligatory. (Contains 2 tables and 2 figures.)