The Giotto spacecraft was decelerated by impacting cometary dust during its flight through the coma of comet P/Grigg-Skjellerup on July 10, 1992. The change in radial velocity determined by two-way Doppler measurements made on July 8 (pre encounter) and July 11 (post encounter) was (-0.4¿0.1) mm/s. An abrupt frequency drop of 138 Hz was observed in the one-way real-time data during the actual encounter phase at 1530:42.8 UT, 6 s after the expected time of closest approach. This frequency discontinuity is attributed to a disturbance in the spacecraft's radio system and is unrelated to the spacecraft deceleration inferred from the more reliable two-way Doppler measurements. The spacecraft spin period changed by (-0.8¿0.1) ms and a nutation of amplitude (0.10¿¿0.01¿) was generated. This relatively large nutation angle, coupled with the very modest deceleration, is most easily generated by a single dust particle. Assuming that the total impacted mass was concentrated in one single large particle, it is possible to determine a range for the total impacted real mass (20--39 mg), the total change in velocity Δv (≂1 mm/s), the momentum enhancement factor &egr; (0 to 2.0), and the impact coordinates on the spacecraft. It is deduced that the particle most likely impacted on the upper end of the spacecraft toward the antenna tripod. No definite solution could be found for impacts on the bumper shield. ¿ American Geophysical Union 1993