The epoch of reionization (EoR) remains a poorly understood cosmic era for the most part. Yet, efforts are still going on to probe and understand this epoch. We present a review of the latest developments in the techniques (especially line-intensity mapping) to study the EoR and try to highlight the contribution of the Indian community in this field. Line-emissions like [Hi]21 cm, Lyman-α, [Cii]158 $\mu$m and their role as tracers in probing the EoR are discussed. While the [Hi]21 cm is an excellent probe of the early inter-galactic medium (IGM), the others are mainly targeted to do an unresolved and large-scale survey of the reionizingsources. Techniques to model these signals include simulations and machine learning approaches along with the challenge to tackle foregrounds or interlopers. We also discuss synergy opportunities among various tracers that we mention. Synergy addresses different aspects of the problem, which is otherwise difficult or impossibleto tackle. They include statistics like cross-power spectrum, cross-bispectrum and other techniques, such as follow-up studies. We present updates on the relevant experiments; these include upper limits on the [Hi]21 cm power spectrum along with some highlights on high-redshift galaxy surveys. Finally, we highlight on what can be improved further within the community: applying machine learning and simulations based on hydrodynamic and radiative-transfer techniques. Next-generation experiments also need to be conceived to address issues, which are currently beyond our reach.

Cosmic dawn (CD) and the epoch of reionization (EoR) are the most important parts of cosmic history during which the first luminous structures emerged. These first objects heated and ionized the neutral atomic hydrogen in the intergalactic medium. The redshifted 21-cm radiation from the atomic hydrogen providesan excellent direct probe to study the evolution of neutral hydrogen (Hi) and thus reveal nature of the first luminous objects, their evolution and role in this last phase transition of the Universe and formation and evolution of the structures thereafter. Direct mapping of the Hi density during the CD–EoR is rather difficult with thecurrent and forthcoming instruments due to stronger foreground and other observational contamination. The first detection of this redshifted Hi signal is expected to be done through statistical estimators. Given the upmost importance of the detection and analysis of the redshifted 21-cm signal, physics of CD–EoR is considered asone of the objective of the upcoming SKA-low telescope. This paper summarizes the collective effort of Indianastronomers to understand the origin of the redshifted 21-cm signal, sources of first ionizing photons, their propagation through the IGM, various cosmological effects on the expected 21-cm signal, various statistical measures of the signal like power spectrum, bispectrum, etc. A collective effort on detection of such signalby developing e stimators of the statistical measures with rigorous assessment of their expected uncertainties, various challenges like that of the large foreground emission and calibration issues are also discussed. Various versions of the detection methods discussed here have also been used in practice with the Giant MeterwaveRadio Telescope with successful assessment of the foreground contamination and upper limits on the matter density in reionization and post-reionization era. The collective efforts compiled here has been a large part of the global effort to prepare proper observational technique, analysis procedure for the first light of the CD–EoR through the SKA-low.

In this review article, we briefly outline our current understanding of the physics associated with the HI 21-cm signal from cosmic dawn. We discuss different phases of cosmic dawn as the ambient gas and the background radiations evolve with the redshift. We address the consequences of several possible heating sources and radiation background on the global 21-cm signal. We further review our present perspective of other important aspects of the HI 21-cm signal, such as power spectrum and imaging. Finally, we highlight thefuture key measurements of the Square Kilometre Array and other ongoing/upcoming experiments that will enlighten our understanding of the early Universe.