Microprocessor technology has permitted the development of a General Earthquake-Observation System (GEOS) useful for most seismic applications. Central-processing-unit control via robust software of system functions that are isolated on hardware modules permits field adaptability of the system to a wide variety of active and passive seismic experiments and straightforward modification for incorporation of improvements in technology. Various laboratory tests and numerous deployments of a set of the systems in the field have confirmed design goals, including: wide linear dynamic range (16 bit/96 dB); broad bandwidth (36 hr to 600 Hz; >36 hr available); selectable sensor-type (accelerometer, seismometer, dilatometer); selectable channels (1 to 6); selectable record mode (continuous, preset, trigger); large data capacity (1.4 to 60 Mbytes); selectable time standard (WWVB, master, manual); automatic self-calibration; simple field operation; full capability to adapt system in the field to a wide variety of experiments; low power; portability; and modest costs. System design goals for a microcomputer-controlled system with modular software and hardware components as implemented on the GEOS are presented. The systems have been deployed for 15 experiments, including: studies of near-source strong motion; high-frequency microearthquakes; crustal structure; down-hole wave propagation; teleseismicity; and earth-tidal strains. These studies have yielded recordings of near-source radiation fields in the frequency band of 1 to 300 Hz with signal resolution greater than 84 dB, documented seismic signals of 80 Hz at distances of 190 km with implications for nuclear detection, provided complete onscale high-resolution recording of several aftershock sequences with signal amplitudes ranging over 180 dB, and records of Earth dilational strain over the period band 0.1 sec to 28 hr, with superimosed radiation fields for nuclear explosions at regional distances and near-source earthquakes. Data sets recorded on the GEOS illustrate the importance of broad bandwidth, high resolution, and wide linear dynamic range for future earthquake studies. Field deployments of a minicomputer system compatible with the GEOS have emphasized the usefulness of portable field computers for experiments using microcomputer-controlled data-acquisition systems.