The discovery of carbon nanotubes has inspired remarkable advances in science and engineering at the nanometer scale. I will describe experiments that demonstrate a powerful approach to control the electronic states in carbon nanotubes. The self-assembly of molecules inside the hollow cores of these tubes is exploited to create a new class of nanotube materials, the properties of which are examined using state-of-the-art scanning tunneling microscopy techniques. The electronic states of composite nanotube-based materials are shown to depend on the interaction between one-dimensional states of carbon nanotubes and the localized orbitals of encapsulated molecules. Such fine-tuning of nanotubes' electronic states has potential applications for exploring the physics of one-dimensional systems, as well as making electronic devices based on these molecules.