ABSTRACT This paper describes the development and initial testing of an instrument to measure population-based public health nursing competencies. Although multiple lists of public health competencies exist, literature review did not elicit a valid instrument that could measure changes in public health nursing competency over time. The public health nursing competency instrument, consisting of 195 measurable activities organized in the framework of the nursing process, was developed. Competency scores of practicing public health nurses significantly increased after a continuing education series, and the instrument was confirmed by experts to be a valid reflection of public health nursing practice. The time required for instrument development exceeded expectations because of the multiple stages of delineating competencies and validating data with national experts.