Research on input processing (IP) examines how second language (L2) learners process the language they encounter in oral or written input, seeking to understand the psycholinguistic principles that guide L2 comprehension and their impact on subsequent acquisition processes. Much of this research involves applying insights from VanPatten’s IP model for the development of more effective pedagogical interventions (processing instruction [PI]). Here, we systematically review and discuss the methodology used in research that either tested the IP model or assessed the effectiveness of PI. Our analysis includes 108 empirical studies that used offline measures of learning (i.e., measuring comprehension after input exposure). We first present the general characteristics of this type of research and then describe key methodological features of IP studies in depth. We conclude with methodological recommendations and suggestions for future research on IP and PI. We hope to provide readers with a better understanding of the strengths and weaknesses of existing research in the hope of contributing to methodological improvements of future research.