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The eukaryotic 40S ribosomal subunit locates the translation initiation codon on an mRNA via the so‐called scanning process that follows 40S binding to the capped 5′ end. This key step in translation is required for the expression of almost all eukaryotic genes, yet the mechanism and dynamics of scanning are unknown. We have performed quantitative studies in vivo and in vitro of the movement of yeast 40S ribosomes along 5′ untranslated regions (UTRs) of different lengths. 40S subunits perform cap‐dependent scanning with high processivity for more than 1700 nucleotides in cells of Saccharomyces cerevisiae. Moreover, the observed rates of expression indicate that scanning is performed by an untethered 40S subunit that has been released from the 5′ cap complex. Unexpectedly, the capability to maintain scanning competence on a long 5′ UTR is more dependent on the Ded1/Dbp1 type of helicase …