Lissencephaly and subcortical band heterotopia are closely related cortical malformations and are true disorders of neuronal migration. The genetic basis of approximately 70% of classic lissencephaly and 80% of typical subcortical band heterotopia is known. Most are due to abnormalities within the LIS1 or DCX genes, with abnormalities ranging from single basepair substitutions to contiguous gene deletions. Understanding the genetic basis of these disorders has led to the elucidation of the molecular and developmental mechanisms that are adversely affected. There is a robust correlation between many of the clinical aspects of lissencephaly or subcortical band heterotopia and the type and location of mutations in the affected gene. Using this knowledge, the clinician can predict with some accuracy which gene is likely to be affected based on the clinical and imaging features. This review answers some of the key questions regarding the genotype-phenotype correlation for lissencephaly and subcortical band heterotopia.