Radio Astronomy in the Early Twenty-First Century

This paper serves as an introduction to the contributions in this Special Issue on ldquoAdvances in Radio Telescopes.rdquo After a very short historical view of the emergence of Radio Astronomy, we refer to earlier IEEE special issues on this subject and mention recent instruments in the domain of millimeter wavelength radio telescopes, developments in very long baseline interferometry and the planned Square Kilometre Array (SKA). After a short discussion of site selection aspects for the new telescopes we conclude with a summary of the major astronomical and astrophysical problems which will be studied by the new instruments described in the following papers.