Biologists have been fascinated for more than 2 centuries about how the nucleus 
in eukaryotes is organised. Certain of the component parts are well known, but 
the overall picture is blurred and often confusing. Small genome species have 
chromosomes in their interphase nuclei disposed in diffuse chromosome 
territories, without any Rabl arrangement, while in large genomes the 
chromosomes run string-like through the nucleus with a Rabl orientation 
following through the cell cycle. What happens in genomes of intermediate size 
is either a bit of both, depending on the tissue being studied, or still remains 
to be determined. The centromeres are the most dynamic and least well understood 
part of the nucleus, subject to rapid evolutionary change and with an epigenetic 
mark based on a special form of histone CENH3. Nonetheless, the centromere 
epigenetic mark has been inherited for millions of years by a process that is a 
complete mystery. Centromeres are involved with the dynamic interactions between 
chromosomes and other parts of the nuclear environment, such as the nuclear 
matrix and inner nuclear membrane, and they also engage with the spindle when 
the order within the nucleus changes during its division. The nucleolus 
organizer regions have likewise posed tantalising problems about their massive 
amplification of rDNA sequences, and how they are regulated and expressed. Some 
of these issues are now becoming clearer with advances in the science and the 
ongoing development of new molecular tools. These developments are discussed in 
this contribution, with particular reference to the centromere and the nucleolus 
organizer.