Low literacy among adolescents and adults worldwide remains a serious problem, even in economically developed countries. The consequences of low reading proficiency levels can be harmful in many ways for both the individuals concerned and their communities in terms of health, political, social and economic outcomes. While large-scale international assessments do assess reading proficiency, the data they provide for the bottom end of the scale are still somewhat undifferentiated. What is of particular concern to scholars and policymakers alike is to better understand the nature of reading difficulties among low-literate adolescents and adults. Addressing this need, the authors of this article present a new integrative process model which takes into consideration reader-related, text-related and task-related factors along different stages of the reading process that can cause reading difficulties. The process model incorporates different traditions of research on low reading proficiency: large-scale assessments, cognitive psychology, and research on developmental precursors of reading comprehension. It enabled the authors to identify core difficulty-generating factors, in particular task and text characteristics relevant in evaluating the difficulty of a reading task and thus in determining whether low-literate readers can solve it. The process model also proved suitable for incorporation into standard-setting practice. The authors demonstrate how the process model provided a framework for developing proficiency level descriptors (PLDs) which were then applied for the purpose of standard setting in a German large-scale assessment, the National Educational Panel Study (NEPS). Their results show that the PLDs were indeed suitable for differentiating between a "low" reading proficiency level and a "functional" reading proficiency level among adolescents and adults.