The 5' untranslated region (5'UTR) is critical in determining post-transcriptional control, which is partly mediated by short upstream open reading frames (uORFs) present in half of mammalian transcripts. uORFs are generally considered to provide functionally important repression of the main-ORF by engaging initiating ribosomes, but under specific environmental conditions such as cellular stress, uORFs can become essential to activate the translation of the main coding sequence. In addition, a growing number of uORF-encoded bioactive microproteins have been described, which have the potential to significantly increase cellular protein diversity. Here we review the diverse cellular contexts in which uORFs play a critical role and discuss the molecular mechanisms underlying their function and regulation. The progress over the last decades in dissecting uORF function suggests that the 5'UTR remains an exciting frontier towards understanding how the cellular proteome is shaped in health and disease.