Big bang nucleosynthesis: Present status

How do we understand the production of the lightest nuclides from H to Li during the first seconds of cosmic time? This article reviews recent developments based on new precision cosmic microwave background measurements from the Planck satellite and observational abundance data. Utilizing updated input on nuclear reactions and the neutron lifetime as well as limits on the baryon density of the Universe obtained from Planck data leads to a number of neutrino flavors.