RNAs (circ RNAs) comprise an emerging new class of endogenous RNAs expressed abundantly by the eukaryotic transcriptome. They are characterized by a covalently closed loop structure, resulting in RNAmolecules that are more stable than linear RNAs. A growing number of studies indicate that circ RNAs play critical roles in human diseases and show great potential as biomarkers and therapeutic targets. The molecular events determined by circ RNAactivity, the circ RNAcode, involve other types of noncoding RNAmolecules, primarily micro RNAs, long noncoding RNAs, and RNA‐binding proteins. Herein, we mainly focus on the circ RNA–micro RNAcode, showing how this relationship impacts the regulation of gene expression in cancer. The emerging roles for circ RNAs in oncogenic pathways highlight new perspectives for the detailed molecular dissection of cancer pathogenesis and, at the same time, offer new opportunities to design innovative therapeutic strategies. Here, we review recent research advancements in understanding the biogenesis, molecular functions, and significance of circ RNAs in cancer diagnosis and treatment.