Models of the glacial isostatic adjustment (GIA) process, which is dominated by the influence of the Late Pleistocene cycle of glaciation and deglaciation, require two fundamental inputs: a history of ice-sheet loading and a model of the variations of viscosity in the Earth's mantle. One of the main methods by which these models may be tested and refined is via comparisons of their relative sea-level history predictions in certain regions to geological inferences based upon past sea level indicators from the same regions. In this context, a region of considerable interest is the East coast of the United States, for which a new dataset of indicators of past relative sea level is now available and which provides a new handle on the GIA problem and its two main inputs. In this presentation, a review of the basics of the GIA process, as well as of the main geological tools by which models of the process can be evaluated, will be performed. Also, an overview of current work involving the incorporation of these new data into our understanding of the effective viscosity of the mantle and of the deglaciation having followed the Last Glacial Maximum will be presented.