The present invention relates to short polypeptides (e.g., fewer than 19 amino acids in length) and longer polypeptides (e.g., 19 or more amino acids in length) having one or more D-amino acids as targeting moieties. These polypeptides, when conjugated to agents (e.g., therapeutic agents or transport vectors) are capable of transporting the agents across the BBB or into particular cell types. In particular, the short polypeptides can include one or more D-amino acids. These compounds are therefore particularly useful in the treatment of neurological diseases or diseases associated with particular cell types, organs, or tissues.
