The present study investigates the understanding of the equals sign among middle school students. The study data were gathered from a total of 433 middle school students (111 in the fifth grade, 106 in the sixth grade, 95 in the seventh grade and 121 in the eighth grade) using a ten-item equals sign test developed by the researcher. The study findings suggest that most students have an operational understanding of the equal sign. The students were most successful in answering true/false number sentences and successful lowest in equivalent equations. It was observed that those who applied a relational strategy to the solving of problems understood the underlying structure of the equation, while those who used an operational strategy were observed to make such errors as "the answer comes next" and "extend the problem".