  The paper is a brief overview of the works by Iosif S. Shklovsky
(1916--1985), carried out over almost 30 years (1955--1985), on the nature of
activity (primarily in the radio frequency range) in nuclei of some galaxies.
  Worthy of note is Shklovsky's pioneering work of 1962, in which he made an
attempt to consider possible evolutionary tracks of extragalactic radio sources
by constructing an analog of the Herzsprung--Russel diagram for stars (radio
luminosity at 160 MHz was taken instead of optical luminosity; total radio size
at the same frequency, as the other parameter). Later works by other authors
are also discussed, where similar diagrams were plotted using a larger
observational material.
  Special attention is paid to the evolution of Shklovsky's views regarding the
possible ways of gas getting into radio galaxies' central regions, followed by
high-velocity ejections of magnetized plasmons from their nuclei. In his
assumptions, Shklovsky was mainly based on the observational data for the
properties of the closest radio galaxy, NGC 4486 (Virgo A, M87), which he
believed to be the same reference standard for extragalactic radio astronomy as
the Crab Nebula for galactic radio astronomy.
  Shklovsky's approach to the recurrence of the activity phenomenon in galactic
nuclei and the one-sided character of radio ejections from them is discussed.
  Modern views on these issues are also briefly considered.

Astrophysics of Galaxies;High Energy Astrophysical Phenomena