The identification and characterization of the exosome complex has shown that 
the exosome is a complex of 3' --> 5' exoribonucleases that plays a key role in 
the processing and degradation of a wide variety of RNA substrates. Advances in 
the understanding of exosome function have led to the identification of numerous 
cofactors that are required for a selective recruitment of the exosome to 
substrate RNAs, for their structural alterations to facilitate degradation, and 
to aid in their complete degradation/processing. Structural data obtained by 
two-hybrid interaction analyses and X-ray crystallography show that the core of 
the exosome adopts a doughnut-like structure and demonstrates that probably not 
all exosome subunits are active exoribonucleases. Despite all data obtained on 
the structure and function of the exosome during the last decade, there are 
still a lot of unanswered questions. What is the molecular mechanism by which 
cofactors select and target substrate RNAs to the exosome and modulate its 
function for correct processing or degradation? How can the exosome discriminate 
between processing or degradation of a specific substrate RNA? What is the 
precise structure of exosome subunits and how do they contribute to its 
function? Here we discuss studies that provide some insight to these questions 
and speculate on the mechanisms that control the exosome.