Inferencing is an important and complex process required for successful reading comprehension. Previous research has suggested instruction in inferencing is effective at improving reading comprehension. However, varying definitions of inferencing is likely impacting how inferencing instruction is implemented in practice and inferencing ability is measured. The goal of this study was, first, to systematically review the literature on inference instruction to compile a list of definitions used to describe inferences, and second, to review textbooks used in instruction and assessments used in research and practice to measure inferencing skills. A systematic literature search identified studies that implemented inferencing instruction with learners across all ages from preschool to adults. After screening and elimination, 75 studies were identified and reviewed for inference definitions, instructional practices, and assessments used. A widely-used reading textbook and two reading comprehension assessments were reviewed for grade 4 (elementary school) and grade 7 (middle school) to connect inferences taught and measured with the identified definitions. Reviewing the 75 studies suggested 3 broad categories of inferences and 9 definitions of specific inference types. Textbook and assessment review processes revealed differences between the types of inference questions practiced and tested. The large variation in inference types and definitions may create difficulties in schools implementing inference instruction and/or attempting to measure students' inference abilities. More alignment between research studies on inference instruction and the textbooks and assessments used in schools to teach and assess inference skills is needed.