Chemotherapy resistance (chemoresistance) is one of the main challenges in cancer treatment, and researchers are investigating the molecular mechanisms involved in chemoresistance for several years. The main mechanisms that are employed by cancer cells to evade the efficacy of chemotherapeutic agents are (1) transported-based molecular mechanisms, (2) non-transporter-based molecular mechanisms, (3) signaling pathways, and (4) epithelial-to-mesenchymal transition. Dysregulation in the expression level of these mechanisms can lead to chemoresistance. MicroRNAs, as regulators of mRNA translation, can cause sensitivity or resistance to chemotherapeutic agents by affecting the expression of these mechanisms. Understanding the expression pattern of microRNAs in the tumor tissues and cancer cells can be beneficial to develop biomarkers for predicting the response of a particular cancer cell to a specific chemotherapeutic drug. Another application for the studies on the role of microRNAs in chemoresistance is designing microRNA antagonists or microRNA mimics to regulate the expression of the downregulated or upregulated proteins involved in chemotherapy resistance. Herein, this chapter defines the molecular mechanisms that participate in resistance to chemotherapy, describes the findings regarding the role of microRNAs in each of these mechanisms, and explains how potential microRNA antagonists or microRNA mimics can help to overcome chemoresistance.