One of the most important protein complexes involved in maintaining correct RNA 
levels in eukaryotic cells is the exosome, a complex consisting almost 
exclusively of exoribonucleolytic proteins. Since the identification of the 
exosome complex, seven years ago, much progress has been made in the 
characterization of its composition, structure and function in a variety of 
organisms. Although the exosome seems to accumulate in the nucleolus, it has 
been clearly established that it is also localized in cytoplasm and nucleoplasm. 
In accordance with its widespread intracellular distribution, the exosome has 
been implicated in a variety of RNA processing and degradation processes. 
Nevertheless, many questions still remain unanswered. What are the factors that 
regulate the activity of the exosome? How and where is the complex assembled? 
What are the differences in the composition of the nuclear and cytoplasmic 
exosome? What is the detailed structure of exosome subunits? What are the 
mechanisms by which the exosome is recruited to substrate RNAs? Here, we 
summarize the current knowledge on the composition and architecture of this 
complex, explain its role in both the production and degradation of various 
types of RNA molecules and discuss the implications of recent research 
developments that shed some light on the questions above and the mechanisms that 
are controlling the exosome.