The principal driving force for the explosive renaissance in liquid chromatography in the last decade can be found in the failure of gas chromatography to accommodate compounds of medium and high molecular weight. However, even liquid chromatography, including its subclass of exclusion chromatography, weakens at very high molecular weights and eventually becomes inapplicable. The primary difficulties stem from unbalanced phase distribution and interfacial adsorption. These problems are largely avoided in field-flow fractionation (FFF). FFF is a methodology resembling chromatography in its dynamical aspects. However, it is a one-phase system, and is therefore, technically, not chromatographic. In FFF, external fields or gradients in one phase replace the partition and adsorption forces that, in chromatography, distribute solute between differential flow regions. Working in one phase, one can virtually eliminate the distortion in phase distribution and the interfacial adsorption that eventually occur with increasing molecular weight in all forms of chromatography. FFF analysis can be extended freely into the polymer and even the particulate range. So far, a billionfold mass range has been explored, roughly from 10 3 to 10 12 daltons. In this paper, the principles, applications, and characteristics of FFF are described; some limitations of exclusion chromatography are noted; and the basic factors involved in extending the tractable molecular weight range are discussed.