The quest for improved time control, to estimate rates of geologic and paleoenvironmental processes, is central to the Earth sciences. Until the middle of this century, geochronology was based solely on macrofossil evolution, providing the largely unmodified definition of geologic stages. In the last few decades, with impetus from the Deep Sea Drilling Project (DSDP) and the Ocean Drilling Program (ODP), microfossil biostratigraphy has become increasingly important. Although new radiometric techniques have resulted in improved absolute age precision, the direct correlation of absolute ages to biozonations (and hence to geologic stage boundaries) remains poor. This is due to the difficulty of finding materials suitable for radiometric dating which can be directly correlated to biozonations. The bridge between biozonations and absolute ages and the interpolation between absolute ages are best accomplished, particularly for Cenozoic and late Mesozoic time, through the geomagnetic polarity timescale (GPTS). The template for Late Jurassic to Quaternary geomagnetic polarity is derived from oceanic magnetic anomalies. As demonstrated by Heirtzler et al. [1968], the GPTS can be constructed using oceanic magnetic anomaly record(s), assuming constant seafloor spreading rate(s), and interpolating between available radiometric ages. In the last quadrennium, there have been outstanding developments in radiometric dating and astrochronology which have greatly improved the absolute age control of the GPTS.
Until recently, the K-Ar chronogram ages compiled by
Mankinen and Dalrymple [1979] were the standard reference for the
age of reversals younger than 5 Ma. In the last few years,
Ar/
Ar techniques have shown that conventional K-Ar
techniques give ages which are very often too young by as much as
10-20%. This is not believed to be due to error in the decay
constant, but rather due to diffusive loss of 
Ar and the problem
of totally degasing sanidine crystals. In addition to the advent
of 
Ar/
Ar techniques, small sample and single crystal U/Pb
zircon techniques with greatly enhanced precision have improved
absolute age control, particularly for the pre-Cenozoic GPTS.
The first astrochronological estimate for the age of the Brunhes/Matuyama boundary to show discrepancy with the standard K-Ar chronogram age (730 ka) was that of Johnson [1982]. Acceptance of an older age (780 ka) for the Brunhes/Matuyama boundary came with isotopic records from more complete Brunhes sections with higher sedimentation rates [ Shackleton et al., 1990]. At about the same time, the matching of sapropel occurrence and carbonate-content cyclostratigraphy to astronomical solutions in Plio-Pleistocene sediments with magnetostratigraphy from southern Italy led to revised ages for reversals in the Gauss and Gilbert [ Hilgen and Langereis, 1989; Hilgen, 1991a,b].