  The Square Kilometre Array (SKA) will offer an unprecedented view onto the
early Universe, using interferometric observations of the redshifted 21cm line.
The 21cm line probes the thermal and ionization state of the cosmic gas, which
is governed by the birth and evolution of the first structures in our Universe.
Here we show how the evolution of the 21cm signal will allow us to study when
the first generations of galaxies appeared, what were their properties, and
what was the structure of the intergalactic medium. We highlight qualitative
trends which will offer robust insights into the early Universe.
