We present a technique to control the length of carbon nanotubes. Individual carbon nanotubes can be locally cut by applying a voltage pulse to the tip of a scanning tunneling microscope (STM). Topographic imaging and STM spectroscopy are subsequently used to investigate the result. The electronic properties of a nanotube can be strongly changed by reducing the size. Current-voltage curves obtained by STM spectroscopy on a 30 nm short tube created from a longer nanotube show a stepwise increase of the current, which is attributed to quantum size effects. ©1997 American Institute of Physics.