A review of the literature was made of the various procedures for sequencing instructional units in individualized instruction programs. Procedures for validating sequences of instructional units, or learning hierarchies, were found to fall into two general categories. One category included procedures based on coefficients of dependence, while the other category contained procedures based on more complete descriptions of the relationships between units of instruction, usually a mathematical model. Procedures based on coefficients were found to provide insufficient information for optimal sequencing of units. Procedures based on mathematical models were also found to be inadequate, but the model-based approach was found to have potential for dealing with the problem of sequencing. Recommended future research emphasizes improving model-based procedures so that they provide more information useful for determining the optimal order of instructional units. (Author/PN)