Mesenchymal stem/stromal cells (MSCs) are known to be the helpers for the healing of tissue damage, often referred to as ambulatory cells. However, MSCs are also recruited by cancer cells to similarly aid in tumor growth and progression. In this review, some of the key steps in cancer progression and metastases are described including the various steps in which MSCs participate and may play important roles. MSCs aid in cancer cells’ ability to evade immune attack, while promoting tumor angiogenesis, even being counter-acting against chemotherapeutics and other drugs used to fight various cancers. Furthermore, MSCs participate in many of the crucial steps in invasion and metastasis, including stimulating the epithelial-mesenchymal transition (EMT) and induction of stem-like properties that allow cancer stem cells to increase their survivability through the circulation. These steps are described in detail. Differences between circulating tumor cells (CTCs) and cancer stem cells (CSCs) are discussed, along with descriptions of the formation of a pre-metastatic niche, the role of exosomes from both cancer cells and MSCs in metastasis and tumor reseeding (self-seeding). More and more, MSCs are being proposed as a promising tumor targeting drug-delivery tool. In order to fulfill this promise, further understanding of the precise roles that MSCs play in the process of cancer metastases must be achieved, in attempting to create remedies that will improve the outcome of available therapeutics.