One of the most important tasks of any cell is to synthesize ribosomes. In eukaryotes, this process occurs sequentially in the nucleolus, the nucleoplasm and the cytoplasm. It involves the transcription and processing of pre-ribosomal RNAs, their proper folding and assembly with ribosomal proteins and the transport of the resulting pre-ribosomal particles to the cytoplasm where final maturation events occur. In addition to the protein and RNA constituents of the mature cytoplasmic ribosomes, this intricate process requires the intervention of numerous protein and small RNA trans-acting factors. These transiently interact with pre-ribosomal particles at various stages of their maturation. Most of the constituents of pre-ribosomal particles have probably now been identified and research in the field is starting to unravel the timing of their intervention and their precise mode of action. Moreover, quality control mechanisms are being discovered that monitor ribosome synthesis and degrade the RNA components of defective pre-ribosomal particles.