During development, each of our cells must resolve its fate, shape and position. Revealing how these decisions are made is critical to understand how mammalian embryos form, yet their real time control in vivo remains unknown. Because fixed specimens cannot capture real time cell dynamics, I have established a cross-disciplinary research program that combines imaging technologies, quantitative and genetic methods, to study cells directly in the living mouse embryo. We have recently showed how transcription factors (TFs) bind to the DNA in single cells of living embryos to control cell fate. We also started to discover some of the mechanisms explaining how cells in the embryo change their shape, using long filopodial protrusions, and how they regulate their mechanical properties to adopt distinct positions within the embryo. Our overriding goal is to exploit the application of our single-cell imaging and quantitative technologies in the living mouse embryo, to reveal how the mechanisms that govern the first changes in cell fate, shape and position are integrated across the entire embryo to ensure normal embryogenesis.