Citation analysis is a useful method for studying a wide range of topics in bibliometrics and the sociology of science. However, many challenges have been made to the validity and reliability of the underlying assumptions, the data, and the methods used in citation studies. This article addresses these issues in three parts. First is a brief review of validity and reliability issues in citation research. Next we explore measurement error in a principal source of journal-to-journal citation data, the Institute for Scientific Information'sJournal Citation Reports. Possible sources of measurement error include discrepancies between citing and cited data, changed or deleted journal titles, aberrant abbreviations, and listing algorithms. The last section is a detailed description of ways to overcome some of the measurement errors. The data and examples are drawn from a journal-to-journal citation study in the fields of Communication, Information Science, and Library Science.