A pulsar timing tutorial and NRAO Green Bank observations of PSR 1257+12.

The steps required in precise timing of pulsars, including pulsars with orbital perturbations, are outlined. Observations of PSR 1257+12 confirm the double periodicity reported by Wolszczan and Fail (1992) with independent data and independent analysis. Observations of PSR 1855+09 compare favorably with a timing model based on independent data (Ryba and Taylor 1991), and thus serve as a clock and system check. The timing of PSR 1620-26, an 11 ms pulsar in the globular cluster M4, show a large period second derivative which, if gravitational indicates a low mass companion or an encounter at distances of 10 - 100 AU.