Neoplastic growth is closely linked to neovascularization, as efficient blood supply is necessary to deliver oxygen and nutrients to a tumour. The development of blood vessels in tumours is modulated by pro- and anti-angiogenic factors. Pro-angiogenic factors include those that regulate remodelling of the extracellular matrix (ECM) and changes in perivascular cell structure, as well as those that promote endothelial cell changes and migration, including but not limited to vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), epidermal growth factor (EGF), platelet-derived growth factor (PDGF), and angiopoietin. Anti-angiogenic factors include thrombospondin, 16kDA N-terminal fragments of prolactin and growth hormone, endostatin, vasostatin, and angiostatin. During tumour growth, the balance is tipped in favour of pro-angiogenic factors. This is known as the angiogenic switch and allows for increased tumour progression, a state where proliferation is favoured over apoptosis. The angiogenic switch may thus be considered as the rate-limiting step in the tumour metastasis pathway. Furthermore, this switch is highly dependent on changes in the tumour microenvironment. The tumour microenvironment continues to increase in significance in angiogenesis research and understanding it holds the key to new and more successful anti-angiogenic cancer therapies.