In this paper we review the concept of organism analysing the main ideas re
lated to it in the context of present biological theories. The discussion i
s focused and developed according to four key issues: individuality, organi
sation, autonomy and reproduction. Once these basic connections are establi
shed, a spectrum of possible entitles that fall under the label 'organism'
is looked over, with special emphasis on limit or controversial cases. The
aim is to see whether they all share a set of common features and, if they
do, why it is so difficult to reach a consensus on the definition of the te
rm. Finally, we try to release somehow the tension between those hierarchic
al schemes proposed to account for life as a global phenomenon and those ap
proaches that take organisms as the central target of (theoretical) biology
, suggesting a possible middle-ground solution open for further research.