Models of the glacial isostatic adjustment (GIA) process enable us to study and understand the impact of the large cycles of glaciation and deglaciation that have characterized the Earth's climate over the past 800,000 years or so on the Earth's surface, in particular in terms of the large variations in sea level that have accompanied them. These models, which require as fundamental inputs a history of ice-sheet loading and a representation of theviscosity of the mantle, can be tested and refined by comparing their predictions to a wide range of observables, among which past relative sea level inferences based upon geological indicators are particularly important. Depending on the region from which they originate,these records provide information about different aspects of of the GIA process. In this presentation, I will present an overview of these various handles on the GIA problem, and focus specifically on how recently available high-quality geological data sets of relative sea level history enable us to further constrain state-of-the-art models.