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
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.