ABSTRACT: This publication describes the rationale and protocol, including design, aims, intervention, and measures, of Families Becoming Healthy Together, a randomized clinical trial examining the effect of a limited RED (non-nutrient-dense, energy-dense) food variety prescription delivered within an 18-month family-based behavioral obesity treatment (FBT) on body mass index (BMI) and habituation rate to RED foods. One hundred fifty-six children (ages: 8-12 y; BMI: ≥ 85th percentile-for-age) and a caregiver (BMI: ≥ 25 kg/m2), both with overweight or obesity, will be randomized to one of two, interventions: FBT or FBT + Variety. All participants will receive 29 sessions of FBT and be prescribed the Traffic Light Diet (1000-1500 kcal/day, ≤ 2 RED food servings/day), and a physical activity goal (≥ 60 min/day [child] or 150 min/week [adult] of moderate-vigorous physical activity (MVPA)). FBT + Variety will also identify two RED foods, a dinner entrée and snack food, and develop meal plans that reduce variety of RED foods by regularly consuming these foods and limiting consumption of other RED foods. Measures of anthropometrics, dietary intake, habituation of salivary response to food cues, and physical activity will be assessed at 0, 6, 12, and 18-months. This study translates a line of basic behavioral research examining how dietary variety influences habituation into a dietary prescription that will be tested within an efficacy trial. It is hypothesized that a novel, limited dietary variety prescription within FBT should promote a faster food habituation rate, reducing energy intake and amplifying long-term weight loss in children.