Although 13 years have passed since the first millisecond pulsar (MSP) was discovered (Backer et al. 1983), it still has the shortest known rotational period (1.56 ms). MSPs are nature’s most stable clocks (Taylor 1991), with timing stabilities that rival atomic clocks on time scales beyond six months (Matsakis & Foster 1996). Specifically, the instantaneous measurement of the period of the original MSP is determined to a precision of ~ 20 attoseconds (10−18 s). With such precision, we are able to predict the pulsar pulse arrival times to a small fraction of the rotational period years into the future. MSPs are powerful sources for use in fundamental astrometry and time keeping applications. Collectively, a population of MSPs distributed around the sky can be used to establish a nearly inertial space-time reference frame (e.g. Foster & Backer 1990). Such a pulsar timing array (PTA) could be used to study drifts in Earth based atomic time scales, perturbation in the planetary ephemerides, relativistic corrections in the solar gravitational potential, and limit the energy density of a stochastic background of gravitational waves from the early universe.