Compaction is the earliest morphogenetic event in mouse embryonic development during which blastomeres increase the size of their cell-cell interfaces. This permits the establishment of apico-basal polarity and thereby the 1st fate specification of the blastomeres: trophectoderm derives from blastomeres with a large apical membrane at the cell-medium interface; whereas blastomeres composing the inner cell mass descend from cells with comparably larger basal cell-cell interfaces. Essential molecules regulating compaction were identified; among them are cell-cell adhesion and cytoskeletal components. However, the precise mechanism by which these molecules drive compaction is not yet understood. In particular, the forces and mechanical properties associated with adhesive and cytoskeletal components that are responsible for the embryo shape change remain uncharacterized. Combining biophysical methods and high-resolution microscopy, we aim to characterize cells mechanical properties and underlying molecules that control compaction. We will monitor, using micropipette aspiration techniques, the contractile and adhesive properties of mouse blastomeres during compaction. In addition, we will quantitatively track changes in cell shape and reorganization of key molecules. With this, we will build a spatio-temporal map of the mechanical properties during compaction. This interdisciplinary and quantitative approach will be key to build a comprehensive model of compaction in early mammalian development.