The purpose of this study is to analyse what kind of conceptions secondary school teachers in mathematics have about equations and how these conceptions are related to the formal definition of the concept of equation. Data was gathered by interviews and questionnaires. Both newly graduated and experienced secondary school teachers were participated in this study. The phenomenographic research approach in order to analyse research outcomes was applied in the investigation. From a phenomenographic analysis of the interview transcripts I found that some patterns could be identified in them and the three qualitatively distinct categories of description about equations could be discerned among the teachers’ conceptions. The research results indicated that equations were apprehended as a procedure, as an answer and as a ‘rewritten’ expression.