Carbon nanotubes are nanometer-diameter cylinders made from rolled up sheets of graphite. Our group creates devices with individual nanotubes as their active elements and then employs electrical measurements and scanned probe techniques to explore their properties. We use both room and low temperature atomic force microscopes to explore the physical and electronic properties of nanostructures. Using these instruments, we can both modify and measure the properties of nanoelectronic devices. For example, we can create defects in nanotube transistors or feel the force associated with the motion of a single electron. We have developed techniques to make electrical contact to individual molecules and make single molecule transistors. The molecules are designed to have specific electronic or vibrational properties that are reflected in the device behavior. For example, we have created a transistor where a single Co atom is the active electronic element. Nanoelectronic devices offer new opportunities to study chemical and biological macromolecules (DNA, etc.). We are developing techniques to detect and manipulate individual macromolecules using nanotube and single-molecule transistors.