MicroRNAs (miRNAs) are small non-coding RNA molecules that critically regulate the expression of genes. MiRNAs are involved in physiological cellular processes; however, their deregulation has been associated with several pathologies, including cancer. In human breast cancer, differently expressed levels of miRNAs have been identified from those in normal breast tissues. Moreover, several miRNAs have been correlated with pathological phenotype, cancer subtype and therapy response in breast cancer. The resistance to therapy is increasingly a problem in patient management, and miRNAs are emerging as novel therapeutic targets and potential predictive biomarkers for treatment. This review provides an overview of the current situation of miRNAs in breast cancer, focusing on their involvement in resistance and the circulating miRNA. The mechanisms of therapeutic resistance regulated by miRNAs, such as the regulation of receptors, the modification of enzymes of drug metabolism, the inhibition of cell cycle control or pro-apoptotic proteins, the alteration of histone activity and the regulation of DNA repair machinery among others, are discussed for breast cancer clinical subtypes. Additionally, in this review, we summarize the recent knowledge that has established miRNA detection in peripheral body fluids as a suitable biomarker. We review the detection of miRNA in liquid biopsies and its implications for the diagnosis and monitoring of breast cancer. This new generation of cancer biomarkers may lead to a significant improvement in patient management.