Principles of 1. the organisation and compartmentalisation of the eukaryotic nuclear genome and 2. of the different processes involved in controlling its activity are outlined and the basic differences to prokaryotic cell systems are emphasized. The special composition and arrangement of the nuclear DNA of eukaryotes is demonstrated in terms of both redundancy classes (unique sequences, sequences of intermediate degrees of repetition, highly repetitive sequences, i.e., "simple sequences") and chromatin and chromosome organisation. The role of the nuclear envelope as an ubiquitous and characteristic structure involved in the compartmentalisation of the nuclear genome and its transcriptional machinery is illustrated. The diversity of the mechanisms of the controls of gene expression in the eukaryotic cells is discussed at different levels: a) chromatin elimination, b) polyploidisation, c) polytenisation, d) gene amplification, e) gene magnification, f) inactivation of genes by complexing with specific proteins and/or protamines, g) transcription, h) complex formation of transcriptional products with specific proteins, i) release of the transcriptional products from the template containing strands, j) processing of newly formed RNA's k) intranuclear degradation of newly formed RNA's, l) nucleocytoplasmic translocation, and m) various forms of RNA containing structures, including masked messenger RNA's and ribosome storages, in the cytoplasm. It is demonstrated that differences in transcriptional activity can be directly visualized and that a direct trnascriptional control of gene activity is indicated to exist at least in some specific cell systems.