Samarium-neodymium (Sm-Nd) isotopic studies can be used to evaluate the history of crustal growth and sometimes to give “crust-formation” ages, which reflect the time of differentiation of crust from the mantle. If, however, a sample is a mixture of material derived from the mantle at different times, Sm-Nd systematics may provide only an estimate of the average time that the material in the sample has been resident in the continental crust. In such cases, Sm-Nd isotopes give no direct information on the timing of crustal formation. These ages can be interpreted as the time of crust-mantle segregation only if supported by other geologic and geochronological information. Misinterpretation can lead to false conclusions about the history of crustal development.