  For various reasons including those related to climate changes, {\em energy}
has become a critical concern in all relevant activities and technical designs.
For the specific case of computer activities, the problem is exacerbated with
the emergence and pervasiveness of the so called {\em intelligent devices}.
From the application side, we point out the special topic of {\em Artificial
Intelligence}, who clearly needs an efficient computing support in order to
succeed in its purpose of being a {\em ubiquitous assistant}. There are mainly
two contexts where {\em energy} is one of the top priority concerns: {\em
embedded computing} and {\em supercomputing}. For the former, power consumption
is critical because the amount of energy that is available for the devices is
limited. For the latter, the heat dissipated is a serious source of failure and
the financial cost related to energy is likely to be a significant part of the
maintenance budget. On a single computer, the problem is commonly considered
through the electrical power consumption. This paper, written in the form of a
survey, we depict the landscape of energy concerns in computer activities, both
from the hardware and the software standpoints.
