The use of sediment accretion rates measured on coastal and estuarine salt marshes as a surrogate for rates of sea-level change rests on the (usually implicit) assumption that marsh maturity is unchanged over the measurement period. This assumption is inherently difficult to test empirically, but a simple mathematical analysis shows that it is true only in a severely restricted case. Under most practical conditions, measured accretion rates either over-estimate or under-estimate the rate of sea-level change. Under certain circumstances, they may indicate an upward trend when the tendency is actually downward. A knowledge of marsh history is critical to the evaluation of such measurements. Sea-level movements estimated from accretion rates are theoretically most reliable in the case of relatively old marshes and comparatively short measurement periods.