Expanding numbers of researchers are focusing on the scale and impact of private supplementary tutoring. Such tutoring is widely called shadow education, since much of its curriculum mimics that of regular schooling. Although shadow education has expanded significantly worldwide and is now recognized to have far-reaching significance, research faces methodological and conceptual challenges. This article focuses on analyses of shadow education data from the Third (or Trends in) International Mathematics and Science Study (TIMSS) and the Programme for International Student Assessment (PISA). An initial problem arises from definitions of shadow education and therefore from research focus. Further challenges arise from the initial phrasing and then translation of items in international questionnaires. The article notes that some studies have been grounded in problematic data, which has led to misleading pictures. Methods and approaches are maturing, but much refinement remains necessary for an adequate understanding of the nature and implications of shadow education.