Addressing malnutrition in Ethiopia is complex because the prevalence of underweight persists as overweight/obesity increases. A limitation in understanding and addressing malnutrition in adults is that body mass index (BMI) is frequently the only measure of nutritional status. Therefore, researchers need tools to assess body composition, measure physical activity, and understand the functional significance of malnutrition. However, a significant limitation to using these tools is that they have not been validated in many low- and middle-income settings. This research evaluated existing bioelectrical impedance analysis (BIA) and skinfold thickness (SFT) prediction equations that calculated fat mass (FM), fat-free mass (FFM), and percent body fat using air displacement plethysmography (ADP). The study participants consisted of 125 females and 129 males residing in Jimma City, Ethiopia. The second aim evaluated the Global Physical Activity Questionnaire (GPAQ) and a 24-hour recall of time use and perceived exertion in measuring the proportion of time spent at moderate and vigorous physical activity (MVPA) using accelerometry. The third aim evaluated the sit-to-stand test (STS), usual gait speed (UGS), and activities of daily living (ADLs) questionnaire as measures of physical function for feasibility, reliability, and validity. The second and third aims took place in rural Tigray, Ethiopia. The study population consisted of females between 18 and 45 years. One existing BIA prediction equation was validated for adult males. No BIA prediction equations were valid for females, and no SFT prediction equations were valid for males or females. New equations were created. The GPAQ was found to have low validity. The 24-hour recall had a fair agreement with accelerometry. The agreement improved by controlling for BMI. STS was a feasible, reliable, and valid measure of physical function. UGS lacked feasibility and reliability. The validity of the ADL questionnaires was inconclusive. This research will provide researchers with better tools to understand and address malnutrition in rural highland Ethiopia. The body composition equations will improve the identification of people who are malnourished. Quantifying physical activity and assessing physical function will enable researchers to understand the causes and consequences of malnutrition, guiding effective interventions to address malnutrition.