The objective and originality of this study was the application of a multi-faceted approach to the investigation of the understanding of the equal sign in primary school children. In the first phase of the study a large proportion of a sample of 126 final grade pupils displayed difficulties in interpreting the concept. In the second phase, all of the primary school mathematics textbooks were reviewed in terms of how they present equalities; a heavy emphasis on the typical operations-equals-answer context was found. In the final phase a sample of 226 children from the final two grades of primary schools was selected. Brief intervention with instruction on the relational meaning of the equal sign, through a specifically designed worksheet containing equations in the non-typical context and some word problems, was found to have a significant positive impact on children’s understanding after a reasonable lapse of time.