The effect of substitution in families of languages, especially an AFL (i.e., abstract family of languages), is considered. Among the main results shown are the following: The substitution of one AFL into another is an AFL. Under suitable hypotheses, the AFL generated by the family obtained from the substitution of one family into another is the family obtained from the substitution of the corresponding AFL. A condition is given for the AFL generated by the substitution closure of a family to be the substitution closure of the AFL generated by the family.