Transformation of normal cells into malignant tumors requires the acquisition of six hallmark traits, e.g., self-sufficiency in growth signals, insensitivity to antigrowth signals and self-renewal, evasion of apoptosis, limitless replication potential, angiogenesis, invasion, and metastasis, which are common to all cancers (Hanahan and Weinberg 2000). These new cellular traits evolve from defects in major regulatory microcircuits that are fundamental for normal homeostasis. The discovery of microRNAs (miRNAs) as a new class of small non-protein-coding RNAs that control gene expression post-transcriptionally by binding to various mRNA targets suggests that these tiny RNA molecules likely act as molecular switches in the extensive regulatory web that involves thousands of transcripts. Most importantly, accumulating evidence suggests that numerous microRNAs are aberrantly expressed in human cancers. In this review, we discuss the emergent roles of microRNAs as switches that function to turn on/off known cellular microcircuits. We outline recent compelling evidence that deregulated microRNA-mediated control of cellular microcircuits cooperates with other well-established regulatory mechanisms to confer the hallmark traits of the cancer cell. Furthermore, these exciting insights into aberrant microRNA control in cancer-associated circuits may be exploited for cancer therapies that will target deregulated miRNA switches.