Purpose: To develop and psychometrically evaluate a questionnaire based on the outcome "Knowledge: Breast-feeding" of the Nursing Outcomes Classification (NOC) to determine the knowledge of parents on breast-feeding., Background: The NOC outcome "Knowledge: Breast-feeding" allows for nurses/midwives to assess the efficacy of interventions aimed to improve the knowledge on breast-feeding in parents thought the clinical interview/observation. However, the use of self-administered questionnaires by patients could facilitate its evaluation., Methods: Two-phased study: (1) Development of the questionnaire based on experts' opinions; (2) Methodological design to assess its psychometric properties., Implications for Nursing Practice: The availability of tools that enable the determination of the knowledge of patients would facilitate nurses/midwives to set objectives, individualize interventions, and measure their effectiveness., (© 2015 NANDA International, Inc.)