A method to functionalize nanotubes by filling could enable the creation of new materials for molecular scale devices. Here we report the synthesis of chains of C-60 molecules inside single-wall carbon nanotubes (SWNTs). C-60 molecules are seen to deposit on the surfaces of SWNTs from the gas phase and to enter the tubes, ostensibly via defects. They subsequently self-assemble into Van der Waals interacting chains, called 'bucky-peapods.' Upon high temperature annealing, encapsulated C-60 molecules coalesce into interior tubes, forming pairs of nested graphene cylinders. The synthesis methods described herein suggest a more general way in which nanotubes may be functionalized.