Long noncoding RNAs (lncRNAs) have gained widespread attention in recent years 
as a potentially new and crucial layer of biological regulation. lncRNAs of all 
kinds have been implicated in a range of developmental processes and diseases, 
but knowledge of the mechanisms by which they act is still surprisingly limited, 
and claims that almost the entirety of the mammalian genome is transcribed into 
functional noncoding transcripts remain controversial. At the same time, a small 
number of well-studied lncRNAs have given us important clues about the biology 
of these molecules, and a few key functional and mechanistic themes have begun 
to emerge, although the robustness of these models and classification schemes 
remains to be seen. Here, we review the current state of knowledge of the lncRNA 
field, discussing what is known about the genomic contexts, biological 
functions, and mechanisms of action of lncRNAs. We also reflect on how the 
recent interest in lncRNAs is deeply rooted in biology's longstanding concern 
with the evolution and function of genomes.