In the last decade the search for galaxies seen within the reionisation era, a period from 500 Myr to approximately 1Gyr after the Big Bang when the Universe transitioned from a neutral to ionised form, has been extremely successful. Exploiting both ground-based (VLT, Keck, Subaru) and space-borne (HST, Spitzer) telescopes, about 1500 galaxies have been discovered in the relevant redshift interval 6 < z < 11. 

Deep surveys have provided valuable demographic data for these early galaxies. They have also provided an initial characterisation of the stellar component, in terms of star formation rates, masses, sizes, etc.  However, to be truly successful such effort must be complemented with information about the internal structure and interstellar medium (ISM) of early galaxies, whose properties are currently unknown.

The ISM plays a fundamental role in galaxy evolution. The diffuse ISM contains the material from which molecular clouds, and thus ultimately stars, form. The Interstellar Radiation Field (ISRF), cosmic rays, and magnetic fields permeate it. By absorbing and radiating the energy produced by stars, it acts as an energy regulator for the galaxy thus playing a pivotal role in galaxy evolution. The ISM is also a repository of metals and dust, which in turn produce emission lines and continuum radiation that represent valuable probes of the physics of early star formation, chemical enrichment and the ionising radiation field.