Modern expression profiling technologies have transformed biological sciences in the past decade, and plant researchers today can chose from a variety of methods that allow the quantification of thousands of transcripts in a single experiment. Using these technologies, the Arabidopsis community has contributed to our understanding of the transcriptional control of developmental programs, be it in entire organs, single cells, or in response to environmental stimuli. Moreover, work in plants has also been instrumental in elucidating the biogenesis and function of small RNAs. With the arrival of commercial whole-genome tiling arrays and cost-effective sequencing technologies, expression profiling will continue to play a central role in plant biology.