Abstract: Ribosome assembly involves rRNA transcription, modification, folding and cleavage from precursor transcripts, and association of ribosomal proteins (Rps). In bacteria, this complex process requires only a handful of proteins in addition to those needed for rRNA transcription, modification and cleavage, while in eukaryotes a large machinery comprising ~200 proteins in the yeast S. cerevisiae has been identified. Furthermore, while the bacterial assembly factors generally produce only cold-sensitive phenotypes upon deletion, most of the eukaryotic assembly factors are essential, comprising ~20% of essential yeast proteins. This review explores recent rapid progress in the structural and functional dissection of the 40S assembly machinery.