The nucleus is an organelle found in eukaryotic cells. Inside its fully enclosed nuclear membrane, it contains the majority of the cell's genetic material. This material is organized as DNA molecules, along with a variety of proteins, to form chromosomes. The development of multicellular organisms and the operation of complex cellular function requires not only perfect orchestration of gene expression but also a precise spatial and temporal control of gene silencing. Gene silencing mechanisms are diverse and exquisitely refined. These mechanisms operate at the transcriptional and post-transcriptional levels, or during meiosis; they use a variety of cellular components such as cis-DNA elements, chromatin modifications, small and non-coding RNAs and specific protein factors responsible for the initiation, operation and readout of silencing.