Teaching competence is a factor that affects the quality of education. The aim of the paper is to analyze the self-perception of in-service secondary school teachers in relation to their professional knowledge. To achieve it, a scale-questionnaire was developed according to the Theory of Reasoned Action AjzenF this questionnaire was applied to a representative and stratified sample of the Valencian Comunity; and data were analyzed with various statistical methods. The results shown a teachers’ high self-perception of professional knowledge, and signifi cant differences for gender, age, experience, or type of institution were found. These results were compared with other researches.