Pseudouridine is the most abundant modification occurring on RNA, yet with the exception of a few well-studied RNA molecules little is known about the modified positions and their function(s). Here, we develop ?-seq, a method for transcriptome-wide quantitative mapping of pseudouridine. We validate ?-seq with synthetic spike-ins and de novo identification of the vast majority of previously reported pseudouridylated positions. ?-seq permits discovery of hundreds of novel pseudouridine modifications in human and yeast mRNAs and snoRNAs. Knockdown and knockout of pseudouridine synthases uncovers the cognate PUSs mediating pseudouridine catalysis at these individual novel sites and their target sequence features. In both human and yeast pseudouridine formation on mRNA depends on both site-specific PUSs – often guided by a specific sequence motif - and snoRNA-guided PUSs. Importantly, upon heat shock in yeast, Pus7-mediated pseudouridylation is induced at >200 sites in diverse mRNAs. Pus7 deletion in yeast leads to decreased recovery from heat shock and decreased RNA levels at otherwise pseudouridylated messages, suggesting a role for pseudouridine in enhancing transcript stability. Pseudouridine stoichiometries in rRNA are highly conserved from yeast to mammals, but are reduced in cells derived from dyskeratosis congenita patients, where the pseudouridine synthase DKC1 is mutated, compared to age matched controls. Our results establish pseudouridine as a ubiquitous and dynamic modification in mRNA, and provide a sensitive, quantitative and transcriptome-wide methodology to address its underlying mechanisms and function. Overall design: Examination of m6A methylation in human Hek293 and A549 cell lines, in human embryonic stem cells (ESCs) undergoing differentiation to neural progenitor cells (NPCs), in OKMS inducible fibroblasts reprogrammed into iPSC, and upon knockdown of factors using siRNAs or shRNAs.