The investigation of processes involved in merging information from different sensory modalities has become the subject of research in many areas, including anatomy, physiology, and behavioral sciences. This field of research termed "multisensory integration’’ is flourishing, crossing borders between psychology and neuroscience. The focus of this chapter is on measures of multisensory integration based on numerical data collected from single neurons and in behavioral paradigms:spike numbers, reaction time, frequency of correct or incorrect responses in detection, recognition, and discrimination tasks. Defining that somewhat fuzzy term, it has been observed that at least some kind of numerical measurement assessing the strength of crossmodal effects is required. On the empirical side, these measures typically serve to quantify effects of various covariates on multisensory integration like age, certain disorders, developmental conditions, training and rehabilitation, in addition to attention and learning. On the theoretical side, these measures often help to probe hypotheses about underlying integration mechanisms like optimality in combining information or inverse effectiveness, without necessarily subscribing to a specific model.