Influenced by work on teacher beliefs, personal epistemology, teachers' knowledge, implicit theories, and motivation, we conducted a two-part investigation exploring preservice and practicing teachers' beliefs about teaching knowledge and teaching ability. Study I qualitatively examined 53 preservice and 57 practicing teachers' beliefs about teaching, in order to: (1) ensure that the voices of teachers were made prevalent in the development of a framework aimed at understanding their beliefs and (2) facilitate the development of a quantitative measure to assess these beliefs. Results indicated that participants valued several aspects of teaching knowledge and held complex beliefs related to the implicit theories of teaching ability. Study I concludes with a framework for developing an instrument to assess teachers' beliefs. Study II describes two instruments developed to assess the beliefs identified in Study 1. Using responses from 351 preservice teachers, exploratory factor analysis procedures identified factors reflective of emergent themes from Study I.