A tumor is a mass of abnormal cells that have uncontrolled growth. The tumor is formed by tumorigenesis, a complex process of converting normal cells into tumor cells under conditions of hypoxia. Tumors can be divided into benign and malignant. The proliferation and progression of a malignant tumor is influenced not only by tumor cells, but also by cells located in the tumor microenvironment (TME), and microRNA (miRNA) also plays an important role in its development. There are two main divisions of miRNAs involved in tumor metabolism, oncomiR and anti-oncomiR. OncomiR are miRNAs that can transform normal cells into tumor cells, and anti-oncomiR can prevent transformation into tumor cells. miRNAs can be considered a key element that affects TMEs cells because they participate in the regulation of gene expression and thus alter TMEs and stimulate the secretion of cytokines. Many years of research into tumor drugs have not yet given the expected results, so further research and understanding of the mechanisms conducive to tumor initiation and progression is needed. Because deregulation of miRNAs is associated with cancer development and progression, circulating miRNAs are considered a new, minimally invasive biomarker and are a potential therapeutic target because the possibility for their manipulation has been proven. In order for miRNA to be used successfully as a therapeutic, it is necessary to bridge the challenge of delivering miRNA to target tissues.