Breast cancer (BC) is a heterogeneous disease that develops into a large number of varied phenotypes. One of the features used in its classification and therapy selection is invasiveness. MicroRNA-21 (miR-21) is considered to be an important element of BC invasiveness, and miR-21 levels are frequently increased in different tumor types compared with normal tissue, including the breast. Experimental and literature research has highlighted that miR-21 was always significantly elevated in every study that included invasive breast carcinomas compared with healthy breast tissue. The main goal of this research was to specify the predominant role of miR-21 in the different phases of BC pathogenesis, i.e. whether it was involved in the early (initiation), later (promotion), or late (propagation, progression) phases. Our second goal was to explain the roles of miR-21 targets in BC by an in silico approach and literature review, and to associate the importance of miR-21 with particular phases of BC pathogenesis through the action of its target genes. Analysis has shown that changes in miR-21 levels might be important for the later and/or late phases of breast cancerogenesis rather than for the initial early phases. Targets of miR-21 (TIMP3, PDCD4, PTEN, TPM1 and RECK) are also primarily involved in BC promotion and progression, especially invasion, angiogenesis and metastasis. miR-21 expression levels could perhaps be used in conjunction with the standard diagnostic parameters as an indicator of BC presence, and to indicate a phenotype likely to show early invasion/metastasis detection and poor prognosis.