Small non-coding miRNAs (microRNAs) are emerging as key factors involved in cancer at all stages ranging from initiation to metastasis. MIRN21 is an miRNA gene that codes for the miR-21 miRNA which has been found to be overexpressed in many tumour samples where it has been analysed. Whereas consistent overexpression of miR-21 in tumours could be suggestive of functional effects of miR-21 in cancer, more in-depth functional studies with miR-21 are demonstrating that mir-21 displays oncogenic activity and can be classed as an oncomir. Extensive efforts are underway to identify the downstream genes and gene networks regulated by miR-21 and to identify the upstream factors that are regulating expression of miR-21. Even though miR-21 is one of the most intensively studied miRNAs, for all miRNAs, our understanding of miRNA signalling pathways is currently in its early stages. The unravelling of such RNA signalling pathways and networks will be key to understanding the role that dysregulated miRNA functioning can play in oncogenic processes.