In this chapter, the data collected from two large international studies conducted by the IEA, namely the 1970-71 Reading Comprehension Study and the 1990-91 Reading Literacy Study ample have been described. Information gathered in eight countries that participated in both studies at itudy. the 14-year-old level, namely Belgium (French), Finland, Hungary, Italy, The Netherlands, New Zealand, Sweden and the U.S., constitute the body of data under consideration. This international evidence is used in the present study in the search for general structural and causal models of reading comprehension that apply across countries. On the one hand, the two studies share certain features with respect to logistics and the type of instruments employed. On the other hand, they differ in aspects such as theoretical assumptions and sampling design. While the data obtained allows examination of the meaningfulness of mean underlying assumptions, the discussion in this chapter has emphasized the consequences of different sampling designs on the interpretation of results. In both studies, large amounts of data were collected. No particular effort was made by the developers of the Reading Literacy Study to ensure a substantial overlap with the instruments and questionnaires that were employed in the Reading Comprehension Study. However, a sufficient number of reading achievement test items as well as questions in student, teacher and, school questionnaires were common to the two studies to provide an interesting opportunity to compare information from the two cross-sectional studies that were conducted 20 years apart. In the following chapter, the concept of simultaneous analysis is employed to examine the factor structures underlying the reading tests in the Reading Comprehension and Reading Literacy Studies. Here, it is of particular interest to examine whether or not: (a) there are several separate skills involved in reading with comprehension at the 14-year-old level; (b) the tests employed on the two occasions measure the same or different reading skills; (c) the hypothesized factor structures remain stable across countries; and (d) it is meaningful to make comparisons of ; student reading achievement at the 14-year-old level across countries.