The advent of fast radioactive ion (RI) beams produced by in-flight
method has brought out great opportunities to investigate the nuclear
structure of nuclei far from stability line. Among several techniques
developed so far for the fast beams, the in-beam gamma spectroscopy
technique has grown up very rapidly to reveal out exotic properties of
unstable nuclei.
In this lecture, I first will give an overview of the present activities
of unstable nuclear physics, and introduce the in-beam gamma spectroscopy
technique. After the introductory part, I will show how exotic results
were obtained by several methods developed, especially for nuclei in
the island-of-inversion region and for the 16C nucleus,
as examples. Based on the "history" of this technical development up
to now, I will discuss perspectives at the future facilities, such as
RIBF, for richer outputs on the nuclear structure.