The lower compacted density of fly ash relative to conventional earthfill recommends its use when a fill or embankment must be constructed over soft, compressible ground. Field evidence to date indicates that compressibility or settlement of compacted fly ash fills is not significant. Long-term field settlements cannot be predicted satisfactorily on the basis of short-term laboratory tests because fly ash exhibits age-hardening or pozzolanic properties. Low-level additions of lime to the fly ash before compaction accentuate this behavior. Frost susceptibility likewise is not as serious a problem in compacted fly ash as might be presumed from its grain size distribution. Gradation alone does not suffice as a criterion of frost susceptibility; pozollanic properties also play an important role. Undesirable frost heave in compacted fly ash can be reduced to tolerable levels by addition of lime or cement.