microRNAs have a principal role in many biological processes, such as gene expression, control and development, as well as cellular processes. In recent years, microRNAs have attracted great interest in the field of toxicology. When organisms are exposed to toxic compounds, microRNAs expression is altered, which affects mRNA transcription and protein translation and causes adverse biological effects. Here, we introduce microRNAs as research tools, the specific expression profile of microRNAs following exposure to different toxicants or diseases that were found to be sensitive biomarkers. Finally, we address several issues related to microRNAs in toxicological studies.