Vasoactive intestinal peptide (VIP) receptors are present in the normal brain as well as periphery, and cancer cells. Three major types of VIP receptors include the VPAC1, VPAC2 and PAC1 receptors. VPAC1 receptors are present in high densities on human lung and breast cancer cells lines and biopsy specimens. Radiolabeled VIP analogues have been developed for imaging of lung and breast cancer. Synthetic VIP receptor antagonists inhibit the proliferation and potentiate the ability of chemotherapeutic agents to cause apoptosis of lung and breast cancer cells. VIP-chemotherapeutic conjugates have been synthesized which bind to VPAC1 receptors and are internalized, resulting in the killing of lung and breast cancer cells. These results suggest that VPAC1 receptors may be molecular targets for diagnosis, prevention and treatment of breast cancer as well as lung cancer.