Star formation efficiency in the first Gyr of cosmic time

In the last decade, the remarkable improvements in instrumental sensitivity at NIR wavelengths have allowed us to probe galaxy formation at just ~400-500 Myr of cosmic time. In particular, the discovery of some incredibly massive galaxies has prompted questions about how such galaxies could build up so quickly. I will present some of the most recent observational results on the assembly of stars in the first Gyr of cosmic time, and how this could be related to the accretion of the hosting dark matter halos.