[en] If a single valence electron of any atom is promoted to a state of high principal quantum number n, the electron experiences an essentially Coulombic potential and behaves in many respects like a highly excited electron in hydrogen. An atom in such a state is known as a Rydberg atom. How this title originated does not seem to be known, but presumably the term is associated with the fact that the spectra of many single-electron atoms are accurately described by Rydberg's hydrogen-like formula. How large n must be for an atom to qualify as a Rydberg atom is vague, but a reasonable working definition is n > 10. Bohr's correspondence principle requires that states with increasingly high quantum numbers assume increasingly classical properties, which might be taken to imply that Rydberg atom research essentially represents a regression from the quantum world to the well-trod world of classical dynamics. However, Rydberg atom research provides a means for changing the scale of atomic interactions by many orders of magnitude, and whenever such a change of scale occurs in physics one can look forward to new discoveries and unexpected phenomena. These introductory lectures summarize some of the discoveries and surprises that have emerged from Rydberg atom research since its modern phase began slightly over ten years ago. 14 references, 14 figures, 1 table