This paper provides a summary and a critique of the empirical literature on elementary teachers' mathematical knowledge. The empirical evidence addresses four major issues surrounding teachers' mathematical knowledge: 1) What is the nature of teachers' content knowledge, particularly with regard to the domain of number? 2) How does teachers' knowledge impact their instructional practice? 3) How does teachers' knowledge impact student learning? and 4) How do teachers develop appropriate mathematical knowledge? For each question, major findings are summarized, exemplars of studies that have addressed the question are provided, and the contribution of these studies to our understanding of teachers' mathematical knowledge is critiqued. The paper concludes that there is no clearly definable body of knowledge that informs teaching; rather, teachers need multiple types of knowledge, each of which is somewhat ill-defined and amorphous. (Contains 65 references.) (ASK)