Dietary choices present an important avenue for promoting food system sustainability. Research suggests that plant-based dietary patterns can have positive health effects and reduce environmental impacts. The Planetary Health Diet was proposed by the EAT-Lancet Commission as a reference healthy and sustainable dietary pattern. A Planetary Health Diet Index (PHDI) was developed for Brazil to measure adherence to Planetary Health Diet recommendations; however, a PHDI has not been developed and evaluated for the United States (U.S.) population. Additionally, relatively few studies have assessed dietary constructs related to sustainability among U.S. adults, including the construct of meat attachment (i.e., a positive bond toward eating meat) and willingness to reduce meat intake. These gaps informed three primary research objectives. First, develop and evaluate the PHDI for the U.S. (PHDI-US). Second, use the PHDI-US to measure American adherence to the Planetary Health Diet. Third, analyze differences in meat attachment and willingness to reduce meat intake by sociodemographic and socioeconomic characteristics in a southern U.S. population. Cross-sectional data from 4,741 participants of the National Health and Nutrition Examination Survey 2017-2018 were used for analyses. Validity and reliability tests were acceptable, with total PHDI-US and Health Eating Index-2015 scores being positively associated; concurrent-criterion validity analyses identifying significantly lower scores among males, everyday smokers, and younger adults; and Cronbach's alpha equaling 0.51. The average PHDI-US score was 38.7 out of 150, indicating that American diets are far from meeting Planetary Health Diet recommendations. Using the Meat Attachment Questionnaire (MAQ), levels of meat attachment, meat intake frequency, and willingness to reduce meat intake and follow a more plant-based diet were compared by gender, educational attainment, income, and age among 328 American adults. The two characteristics most associated with differences in MAQ scores were age and educational attainment, with younger adults and those with higher educational attainment having lower MAQ scores and greater willingness to follow a more plant-based diet. The results of these studies can help improve the design of interventions by providing a tool to quantitatively measure American adherence to the Planetary Health Diet and identifying that younger adults and those with higher educational attainment may be more receptive to adopting sustainable dietary patterns. / Doctor of Philosophy / Dietary choices present an important avenue for promoting food system sustainability. Research suggests that plant-based dietary patterns can have positive health effects and reduce environmental impacts. The Planetary Health Diet is a recommended healthy and sustainable dietary pattern. A Planetary Health Diet Index (PHDI) was developed for Brazil to measure adherence to Planetary Health Diet recommendations; however, a PHDI has not been developed and evaluated for the United States (U.S.) population. Additionally, relatively few studies have assessed dietary constructs related to sustainability among U.S. adults, including the construct of meat attachment (i.e., a positive bond toward eating meat) and willingness to reduce meat intake. These gaps informed three research objectives. First, develop and evaluate the PHDI for the U.S. (PHDI-US). Second, use the PHDI-US to measure American adherence to the Planetary Health Diet. Third, analyze differences in meat attachment and willingness to reduce meat intake by sociodemographic and socioeconomic characteristics in a southern U.S. population. Data from 4,741 participants of the National Health and Nutrition Examination Survey 2017-2018 were used for analyses. Validity and reliability tests were acceptable, demonstrating that the PHDI-US can consistently measure dietary quality and sustainability in accordance with Planetary Health Diet recommendations. The average PHDI-US score was 38.7 out of 150, indicating that American diets are far from meeting Planetary Health Diet recommendations. Using the Meat Attachment Questionnaire (MAQ), levels of meat attachment, meat intake frequency, and willingness to reduce meat intake and follow a more plant-based diet were compared by gender, education level, income, and age among 328 American adults. Younger adults and those with a higher level of education had lower MAQ scores and greater willingness to follow a more plant-based diet. The results of these studies can help improve research studies by providing a tool to measure how closely American diets are meeting Planetary Health Diet recommendations and identifying that younger adults and those with a higher level of education may be more receptive to adopting sustainable dietary patterns.