Recent evidence demonstrates that mathematics knowledge in the early years is an important contributor to students' learning and later achievement in mathematics. As early mathematics education has assumed heightened importance, the quality of early mathematics teaching and learning experiences has attracted global attention. Several researchers have designed and used a number of instruments to measure and portray the quality of mathematics teaching in early childhood classrooms. As this body of work is growing fast, it is imperative to explore similarities and differences among the existing research tools that aim to measure the quality of early mathematics instruction. This paper takes a step towards exploring such similarities and differences by reviewing six observation instruments designed to measure early mathematics teaching quality. Therefore, the aim of this paper is twofold: (1) to provide an up-to-date overview by examining these measures' theoretical bases, foci, and psychometrics; and (2) to examine the similarities and differences between the reviewed tools.