  We review the history and status of the famous classification of
extraterrestrial civilizations given by the great Russian astrophysicist
Nikolai Semenovich Kardashev, roughly half a century after it has been
proposed. While Kardashev's classification (or Kardashev's scale) has often
been seen as oversimplified, and multiple improvements, refinements, and
alternatives to it have been suggested, it is still one of the major tools for
serious theoretical investigation of SETI issues. During these 50+ years,
several attempts at modifying or reforming the classification have been made;
we review some of them here, together with presenting some of the scenarios
which present difficulties to the standard version. Recent results in both
theoretical and observational SETI studies, especially the G-hat infrared
survey (2014-2015), have persuasively shown that the emphasis on detectability
inherent in Kardashev's classification obtains new significance and freshness.
Several new movements and conceptual frameworks, such as the Dysonian SETI,
tally extremely well with these developments. So, the apparent simplicity of
the classification is highly deceptive: Kardashev's work offers a wealth of
still insufficiently studied methodological and epistemological ramifications
and it remains, in both letter and spirit, perhaps the worthiest legacy of the
SETI "founding fathers".
