Colorectal cancer (CRC) is a leading cause of cancer related death. Colon cancer stem cells (CCSCs) play important roles in CRC tumorigenesis and metastasis. The role of non-coding RNAs such as microRNAs and long non-coding RNAs (lncRNAs) in regulating cancer progression and stem cell renewal and differentiation are being increasingly appreciated. The aim of my study is to understand how non-coding RNAs regulate CRC initiation and metastasis. In Chapter 1, we identified that Lnc34a, a previously unidentified LncRNA, is enriched in CCSCs to create a spatial imbalance in microRNA miR-34a expression, leading to the initiation of asymmetric cell division. Lnc34a recruits Dnmt3a via PHB2 and HDAC1 to methylate and deacetylate the miR-34a promoter simultaneously, hence epigenetically silencing miR-34a expression independent of its upstream regulator p53. Lnc34a levels affect CCSC self-renewal and colorectal cancer (CRC) growth in xenograft models. Lnc34a is upregulated in late-stage CRCs, contributing to epigenetic miR-34a silencing and CRC proliferation. In Chapter 2, we describe that miR-34a regulates asymmetric division of normal intestinal stem cells (ISCs). Proinflammatory stress triggers asymmetric division of ISCs that normally undergo symmetric division. Silencing of miR-34a in ISCs inhibits asymmetric division and increases inflammation-induced ISC proliferation. These findings suggest that miR-34a provides a safeguard mechanism against excessive stem cell proliferation under inflammation, which is common during tumorigenesis. In Chapter 3, we demonstrate that miR-34a deficiency leads to colon tumorigenesis after Citrobacter rodentium infection. miR-34a targets both immune and epithelial cells to restrain inflammation-induced reparative regeneration. miR-34a targets the Interleukin 6 receptor (IL-6R) to suppress T helper 17 (Th17) cell differentiation, targets the Interleukin 23 receptor (IL-23R) to block Th17 cell expansion, targets chemokine CCL22 production to hinder Th17 cell recruitment to the colon epithelium, and targets the Interleukin 17 receptor D (IL17RD) in colon stem cells to inhibit IL17-induced stem cell proliferation. This study highlights the importance of microRNAs in protecting tissue integrity during pro-inflammatory response despite their lack of function in regular tissue homeostasis. In Chapter 4, we show that the microRNA miR-1269a promotes CRC metastasis and forms a positive feedback loop with TGF-β signaling. miR-1269a is upregulated in late-stage CRCs and strongly associated with risk of CRC relapse and metastasis. Consistent with clinical observations, miR-1269a significantly increases the ability of CRC cells to invade and metastasize in vivo. TGF-β activates miR-1269 via Sox4, while miR-1269a enhances TGF-β signaling by targeting Smad7 and HOXD10, hence forming a positive feedback loop to promote metastasis. Stage II CRC patients with high miR-1269a expression in resected tumors have significantly higher rate of relapse and worse prognosis. Our findings suggest that miR-1269a is a potential marker to guide adjuvant chemotherapy decisions for CRC patients and a potential therapeutic target to deter metastasis.