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In the International System of Units (SI), the kilogram is the last base unit to be defined in terms of an artifact, a century-old platinum-iridium alloy cylinder. This talk will describe one effort towards a new definition of the kilogram in terms of invariant quantities, the NIST (National Institute of Standards and Technology) Electronic Kilogram project. The project uses a watt balance which measures the ratio of mechanical to electrical work, linking the meter, the artifact kilogram, and the second to the practical realizations of the ohm and the volt derived from the quantum Hall and the Josephson effects. In 1998, the NIST watt balance set an upper limit on the drift rate of the artifact kilogram of 2x10-8/yr (PRL Sept. 21, '98). By using the theoretical values for the Josephson and von Klitzing constants, the same results yield an SI determination of Planck's constant with a combined relative uncertainty of 8.7x10-8, the most accurate determination to date.