In the present chapter we introduce theoretical and empirical approaches to the construct of diagnostic competence within the COACTIV research program. We report eight conceptualizations and operationalizations of diagnostics skills and add in addition three constructs that seem to be close to diagnostic skills. Correlational analyses reveal only moderate and unsystematic relationships. However, our analyses showed the expected differences between school types. The chapter concludes with structural equation models in which the predictive validity of diagnostic skills for mathematical achievement of students is analyzed (both with black box and with mediation models).