Nurse educators worldwide are charged with preparing a workforce to deliver competent genetic and genomic based care. In the United States, a set of broadly endorsed competencies (the Essentials) define genetic and genomic competency for nurses. Certain foundational knowledge, defined as genomic literacy, is necessary for nurses to achieve genomic competency. The level of genomic literacy among nursing students is unknown, and no validated measure of that knowledge is available. Concept inventories are research-based assessments that look like multiple-choice tests. Designed to distinguish students who understand basic concepts from those who do not, concept inventories are increasingly used in science and technology education although none have been developed for nursing. The purpose of this instrument development study was to design, test, and conduct initial psychometric evaluation of a tool to measure conceptual knowledge underlying the Essentials. Key concepts were extracted from the competencies and validated with 104 genetic nurse experts. Student supplied response surveys of 96 to 134 baccalaureate nursing students elicited textual responses to open-ended questions about the key concepts. Analysis of 6,000 textual responses identified the level of understanding of key concepts and common misconceptions. A 55-item draft inventory was constructed, utilizing misconceptions as item distractors. Cognitive think-aloud interviews with 15 nursing students informed item revision. The revised inventory was pilot tested with 238 baccalaureate nursing students. Psychometric analysis informed inventory reduction, retaining the most psychometrically robust items while preserving the initial content domain. The product of this study is a 31-item beta version of the Genomic Nursing Concept Inventory (GNCI © Ward 2011). The inventory's psychometric characteristics are unknown pending further testing; however, estimates of scale performance are promising. Estimated scale difficulty is 53%, with item difficulty of .26 to .83. Average item-total correlation is .33, indicating adequate discriminatory power, and Cronbach's alpha is .804, suggesting sufficient reliability. Although the GNCI requires further validation and large scale testing, it holds great promise to provide a reliable and valid ruler by which to measure genomic literacy. Such a ruler is required to inform curriculum and course design and evaluate outcomes in genomic nursing education.