The chapter discusses the role of matrix metalloproteinases (MMPs) in tumor metastasis, a complex process involving multiple steps such as tumor cell escape, invasion, angiogenesis, and establishment of metastatic foci. MMPs are crucial in degrading the extracellular matrix (ECM) and facilitating tumor cell migration and invasion. The review highlights the extensive experimental and observational data supporting the critical roles of MMPs in these processes, particularly in tumor-induced angiogenesis, which is highly dependent on MMP activity, especially MMP-9. However, the involvement of MMPs in other stages of metastasis, such as tumor cell escape, intravasation, and extravasation, has received less attention, leading to insufficient or controversial data. The chapter also reviews the dual nature of MMPs, where they can exhibit both pro-metastatic and anti-metastatic roles depending on their context and experimental conditions. This duality is a major focus of the review. Additionally, the biological complexity of metastasis is emphasized, requiring complex experimental systems, including in vivo animal models, to study secondary site colonization. The strong correlations between altered MMP expression and poor prognosis in various cancers are discussed, with elevated levels of MMPs like MMP-1, -2, -3, -7, -9, -13, and -14 associated with poor tumor differentiation, invasive stage, and shorter survival. However, rare examples of negative correlations, such as reduced MMP-8 expression in metastatic breast cancer cells, are also noted. The role of soluble MMPs and their interaction with natural inhibitors like TIMPs is also explored, highlighting the unclear functional contributions of plasma MMPs to malignant progression. Overall, the chapter provides a comprehensive overview of the multifaceted roles of MMPs in tumor progression and metastasis, emphasizing the need for further research to fully understand their complex functions.The chapter discusses the role of matrix metalloproteinases (MMPs) in tumor metastasis, a complex process involving multiple steps such as tumor cell escape, invasion, angiogenesis, and establishment of metastatic foci. MMPs are crucial in degrading the extracellular matrix (ECM) and facilitating tumor cell migration and invasion. The review highlights the extensive experimental and observational data supporting the critical roles of MMPs in these processes, particularly in tumor-induced angiogenesis, which is highly dependent on MMP activity, especially MMP-9. However, the involvement of MMPs in other stages of metastasis, such as tumor cell escape, intravasation, and extravasation, has received less attention, leading to insufficient or controversial data. The chapter also reviews the dual nature of MMPs, where they can exhibit both pro-metastatic and anti-metastatic roles depending on their context and experimental conditions. This duality is a major focus of the review. Additionally, the biological complexity of metastasis is emphasized, requiring complex experimental systems, including in vivo animal models, to study secondary site colonization. The strong correlations between altered MMP expression and poor prognosis in various cancers are discussed, with elevated levels of MMPs like MMP-1, -2, -3, -7, -9, -13, and -14 associated with poor tumor differentiation, invasive stage, and shorter survival. However, rare examples of negative correlations, such as reduced MMP-8 expression in metastatic breast cancer cells, are also noted. The role of soluble MMPs and their interaction with natural inhibitors like TIMPs is also explored, highlighting the unclear functional contributions of plasma MMPs to malignant progression. Overall, the chapter provides a comprehensive overview of the multifaceted roles of MMPs in tumor progression and metastasis, emphasizing the need for further research to fully understand their complex functions.