Circular RNAs (circRNAs) in animals are an enigmatic class of RNAs with unknown function. To systematically explore circRNAs, we sequenced and computationally analyzed human, mouse and nematode RNA. We detected thousands of well-expressed, stable circRNAs, with oftentimes tissue/developmental stage specific expression. Sequence analysis suggested important regulatory functions for circRNAs. Indeed, we discovered that human circRNA CDR1as is densely bound by miRNA effector complexes and harbors 63 conserved binding sites for the ancient miRNA miR-7. Further analyses indicated that CDR1as functions to bind miR-7 in neuronal tissues. Human CDR1as expression in zebra fish impaired midbrain development similar to knocking down miR-7, suggesting that CDR1as is a miRNA antagonist with a miRNA binding capacity ten times higher than any other known transcript. Together, our data provide evidence that circRNAs form a large class of post-transcriptional regulators. Numerous circRNAs form by head-to-tail splicing of exons, indicating previously unrecognized regulatory potential of coding sequences. PARCLIP was performed as in Hafner et. al Cell 2010 with HEK293 cell lines stably expressing HIS/FLAG/HA-tagged AGO1 or AGO2. We used 4-thiouridine (4SU) to enhance the crosslink and generated cDNA libraries.