Circular RNAs (ecircRNAs) are abundant, stable, conserved, and nonrandom products of RNA splicing. This study identifies over 25,000 distinct ecircRNAs in human fibroblasts, with 14.4% of actively transcribed genes producing ecircRNAs. These RNAs are more stable than associated linear mRNAs and are enriched by RNase R digestion. EcircRNAs are found in precisely orthologous locations in murine testis RNA, and paralogous kinases HIPK2 and HIPK3 produce abundant ecircRNAs. These ecircRNAs contain ALU repeats and are not bound to ribosomes. They can be degraded by siRNAs and may act as competing endogenous RNAs. Bioinformatic analysis reveals that ecircRNAs are flanked by long introns containing complementary ALU repeats. These data show that ecircRNAs are abundant, stable, conserved, and nonrandom products of RNA splicing that could be involved in gene expression regulation. The study also identifies that ecircRNAs are predominantly cytoplasmic and highly stable. They are not associated with ribosomes and can be targeted by siRNA. EcircRNAs are not randomly distributed in the genome and are more likely to contain ALU repeats. The study suggests that ecircRNAs may regulate gene expression through mechanisms such as affecting translation, RNAi, or sequestration of RNA-binding proteins. The findings indicate that ecircRNAs are a common and abundant form of noncoding RNA.Circular RNAs (ecircRNAs) are abundant, stable, conserved, and nonrandom products of RNA splicing. This study identifies over 25,000 distinct ecircRNAs in human fibroblasts, with 14.4% of actively transcribed genes producing ecircRNAs. These RNAs are more stable than associated linear mRNAs and are enriched by RNase R digestion. EcircRNAs are found in precisely orthologous locations in murine testis RNA, and paralogous kinases HIPK2 and HIPK3 produce abundant ecircRNAs. These ecircRNAs contain ALU repeats and are not bound to ribosomes. They can be degraded by siRNAs and may act as competing endogenous RNAs. Bioinformatic analysis reveals that ecircRNAs are flanked by long introns containing complementary ALU repeats. These data show that ecircRNAs are abundant, stable, conserved, and nonrandom products of RNA splicing that could be involved in gene expression regulation. The study also identifies that ecircRNAs are predominantly cytoplasmic and highly stable. They are not associated with ribosomes and can be targeted by siRNA. EcircRNAs are not randomly distributed in the genome and are more likely to contain ALU repeats. The study suggests that ecircRNAs may regulate gene expression through mechanisms such as affecting translation, RNAi, or sequestration of RNA-binding proteins. The findings indicate that ecircRNAs are a common and abundant form of noncoding RNA.