Our goal is to reveal how mammalian cells resolve their fate and interactions during the earliest stages of life. Because fixed specimens cannot capture cell dynamics in vivo, we established imaging techniques to study cells directly in live mouse embryos. We recently showed how transcription factors search and bind DNA to determine the first cell fates of the embryo. We discovered that differences in the binding of Sox2 to specific DNA sites appear as early as the 4-cell stage, are regulated by histone methylation and predict cell fate. We then showed that as cell choose their initial fates, they also extend long filopodia protrusions to pull their neighbor cells closer, revealing a mechanism for embryo compaction and polarization. Finally, we developed membrane segmentation and laser ablation methods to show how mechanical forces generated by the acto-myosin cortex drive the formation of the pluripotent inner mass of the embryo.