Explore the physics and cosmology of 21cm radiation in this comprehensive lecture by Somnath Bharadwaj. Delve into key concepts such as spin temperature, surface brightness, blackbody radiation, and cosmological expansion. Examine radiative transfer processes and the thermal sources of 21cm radiation. Gain insights into the 21-cm brightness temperature, Einstein coefficients, absorption coefficient, and optical depth. Learn how these fundamental principles contribute to our understanding of the early universe and large-scale structure formation. Part of the "Cosmology - The Next Decade" program at the International Centre for Theoretical Sciences, this lecture provides essential knowledge for researchers and students interested in cutting-edge cosmological observations and theories.