AbstractWhile the cell nucleus was described for the first time almost two centuries ago, our modern view of the nuclear architecture is primarily based on studies from the last two decades. This surprising late start coincides with the development of new, powerful strategies to probe for the spatial organization of nuclear activities in both fixed and live cells. As a result, three major principles have emerged: first, the nucleus is not just a bag filled with nucleic acids and proteins. Rather, many distinct functional domains, including the chromosomes, resides within the confines of the nuclear envelope. Second, all these nuclear domains are highly dynamic, with molecules exchanging rapidly between them and the surrounding nucleoplasm. Finally, the motion of molecules within the nucleoplasm appears to be mostly driven by random diffusion. Here, the emerging roles of several subnuclear domains are discussed in the context of the dynamic functions of the cell nucleus. Mol. Reprod. Dev. 77: 19–28, 2010. © 2009 Wiley‐Liss, Inc.