‘Particles and fields’ is an expression that describes the type of study upon which the scientific part of the U.S. space effort is primarily focused. The observations are, for the most part, made by analyzing information that is carried by a weak radio signal (telemetry) which is transmitted from a spacecraft. The influences of particles and fields of an object in space on the spacecraft, its instruments, and even on the radio signal form the basis of this analysis. The level of sophistication and, indeed, the scientific yield of a ‘particles’ and ‘fields’ study is clearly evident in the recent reports of data analysis of the Pioneer Saturn results (Science, 207 (4429), 400-453,1980). By analysis of an 8-watt radio signal at S band (approximately 2.2 MHz), Saturn's rings were observed, imaged, and analyzed. New rings were found, as was a new satellite (possibly one that had been observed before but ‘lost,’ and its existence thus confirmed by Pioneer). The masses of the rings, many of the satellites, and the main body of Saturn were determined. The temperatures, heat fluxes, and gravitational and magnetic fields were measured and, where possible and appropriate, mapped. It was found that the magnetic field of Saturn, which was not really known before the mission, has a value at the equator of 0.2 gauss (G), which corresponds to a magnetic moment of 4.3×1028 G-cm3. Saturn's magnetic field at the cloud tops is thus slightly smaller than the earth's, even though Saturn's radius is so much larger (by almost a factor of 10; Rs = 60,000 km). The magnetic field has mostly a dipole character, like Jupiter's, in opposite polarity to the earth's, but it was discovered that Saturn's dipole field axis almost coincides with the rotational axis (a tilt of less than 1°, compared with 10° to 20° for the earth and Jupiter). This small tilt angle introduces difficulties in accurately determining Saturn's rotational period.