Neurons utilize mRNA transport and local translation as a means to influence development and plasticity. The molecular mechanisms for this mRNA sorting involve the recognition of cis-acting sequences by distinct mRNA binding proteins that have a dual role, acting in both mRNA transport and translational regulation. Other proteins play a part in the assembly of messenger ribonucleoprotein complexes into transport granules. mRNA binding proteins are crucial targets of phosphorylation signals that regulate local translation. Fragile X syndrome and spinal muscular atrophy have emerged as two genetic neurological diseases that could result, in part, from impaired assembly, localization, and translational regulation of these messenger ribonucleoproteins.