Although since the late 1950s it has been recognized that neutral hydrogen in the IGM may be directly detectable in emission or absorption against the CMB radiation at the frequency corresponding to the redshifted neutral hydrogen 21 cm line providing a direct probe of the era of cosmological reionization, only now radio interferometers such as LOFAR, MWA and PAPER are delivering the first data. At present we only have available observations of the last stages of the process and of the global amount of electrons produced during reionization, but we are lacking any information on the reionization history and its sources. The 21cm line from neutral hydrogen in the IGM would provide invaluable information on the distribution and evolution of HI and would allow, for the first time, to map the temporal evolution of the reionization process (Ciardi & Madau 2003; Di Matteo, Ciardi & Salvaterra 2004; Salvaterra et al. 2005; Valdes et al. 2006; Ciardi & Salvaterra 2007, Maio, Ciardi & Koopmans; Ciardi et al. 2013; Ciardi et al. 2015; Kakiichi et al. 2017; Zackrisson et al. 2019). An increasing attention is being devoted to using the 21cm line in combination with observations in different frequency bands. This would likely (i) help to reduce systematic effects, (ii) confirm the cosmological origin of the 21cm signal, and (iii) provide additional insight into the properties of high-z sources. Investigations have been made on cross-correlating the 21cm line with Ly-α Emitters (Wiersma et al. 2013; Vrbanec et al. 2016), CMB radiation (Ma et al. 2018), X-ray background (Ma, Ciardi, Eide & Helgason 2018), metal emission lines (Moriwaki, Yoshida, Eide & Ciardi 2019).
I am a core member of the LOFAR Epoch of Reionization Key Science Project (e.g. Jelic et al. 2008; Harker et al. 2010; Zaroubi et al. 2012; Ciardi et al. 2013; Jensen et al. 2013; Yatawatta et al. 2013; Wiersma et al. 2013; Patil et al. 2014; Ciardi et al. 2015; Vedantham et al. 2015, Vrbanec et al. 2016; Patil et al. 2017; Gehlot et al. 2019), and MPA has also contributed to the LOFAR project building one of the international stations.