The interplay between abnormalities in genes coding for proteins and microRNAs (miRNAs) has been among the most exiting yet unexpected discoveries in oncology over the last decade. The complexity of this network has redefined cancer research as these molecules produced from what was once considered “genomic trash”, have shown to be crucial for cancer initiation, progression, and dissemination. Naturally occurring miRNAs are very short transcripts that never produce a protein or amino acid chain, but act by regulating protein expression during cellular processes such as growth, development and differentiation at the transcriptional, post-transcriptional and/or translational level. In this review article we present miRNAs as ubiquitous players involved in all cancer hallmarks. We also describe the most used methods to detect their expression, which have revealed through gene expression studies the identity of hundreds of miRNAs dysregulated in cancer cells or tumor microenvironment cells. Furthermore, we discuss the role of miRNAs as hormones and as reliable cancer biomarkers and predictors of treatment-response. Along with this, we explore current strategies in designing miRNA-targeting therapeutics, as well as the associated challenges that research envisions to overcome. Finally, we introduce a new wave in molecular oncology translational research, the study of long non-coding RNAs.