RNA interference (RNAi) is a post-transcriptional gene silencing process elicited by double-stranded RNA, such as micro-RNA (miRNA) and small interfering RNA (siRNA). They are 18-25 nucleotide long, small non-coding RNAs acting as critical regulators in eukaryotic genome expression. They play an important role in regulating a wide range of biological processes such as cell cycle control, differentiation, aging and apoptosis. However, their role in supporting skeletal development and bone homeostasis is still poorly understood. Osteoporotic fractures constitute a tremendous and growing problem in our ageing populations, with an annual incidence of approximately 60000 osteoporotic fractures in Sweden. Osteoporosis is referred as the “Silent epidemic” because bone loss is gradual and a basically symptomless development until a fracture occurs. Results presented in this thesis provide a novel insight into crucial roles of   miRNAs in regulating bone homeostasis. The initial aim for the thesis was to perform global miRNA expression profiling in human bone cells, and to correlate these levels to global mRNA levels. We identified and functionally characterized several miRNAs that were differentially expressed and acted in important bone signaling pathways such as the Wnt and BMP pathways. These miRNAs included hsa-miR-29b, hsa-miR-30c2 and hsa-miR-125b, which we found targeting genes highly relevant to bone metabolism e.g. COL1A1, SPARC, RUNX2, BGLAP and FRZB. Thereafter, the effect on the microRNAome upon external stimuli (e.g., Dexamethasone and Parathyroid hormone) was assessed by SOLiD sequencing. We observed a substantial difference in the expression of miRNAs between PTH and DEX treated cells. Understanding the changes in miRNAome in human bone cells under different conditions could provide new insight in bone remodeling, specifically differentiation and functional properties of osteoblasts. Based on these studies, we furthermore identified Dlx5 as potential common target of miR-203 and miR-320b and these miRNAs negatively regulate BMP-2-induced osteoblast differentiation. To activate the RNAi pathway, siRNA or miRNA molecules must be conveyed into the cytoplasm of target cells. Since challenges in cellular delivery of these small silencing RNA molecules so far have limited their clinical utility, we developed a new siRNA design that demonstrates a novel carrier-free cellular delivery. This development could potentially have a major impact in RNAi therapeutics. In conclusion, this thesis provides novel insight of miRNAs that play a major role in the regulation of bone remodeling and differentiation and functional properties of osteoblasts. Our findings may have diagnostic and/or therapeutic implications in disorders of bone metabolism.