With continued expansion of clean indoor air legislation, private spaces such as homes and cars are gaining increased attention as significant sources of secondhand smoke (SHS). This study examines the establishment and enforcement of smoking rules in family cars. Qualitative interviews (N=136) were conducted with Black and White families in rural Georgia. Participating families had a young adolescent in the home and included households with all nonsmokers, a mix of smokers and nonsmokers, and all smokers. Common car smoking rules included no smoking allowed at any time, smoking allowed if a window is cracked, and no smoking allowed if children or nonsmokers are in the car. Major reasons for rules included protecting children and nonsmokers from SHS, aversion to the smell, and the stifling nature of SHS in cars. Damage to the vehicle from ashes and burns was another reason for smoking rules, particularly among families with smokers. Many families had never discussed car smoking rules. Families with car smoking rules were generally able to enforce them without difficulty, although smokers responded with a range of reactions from acceptance to anger. Families would consider a total ban if the smoker in their family quit or, for a few, if they purchased a new car. These findings have implications for designing intervention strategies to promote smoke-free cars and help family members to negotiate and enforce car smoking rules.