Tumour heterogeneity, or the presence of different cells with varying genetic makeup within a single tumour, is an important factor to consider when developing personalized cancer treatments. To describe this diversity, other terms may also be used, such as intratumoral heterogeneity or clonal heterogeneity. The tumor microenvironment, including the presence of immune cells or fibroblasts, can also contribute to heterogeneity. Morphological heterogeneity describes the physical appearance of cells, while molecular heterogeneity refers to differences in gene expression or mutations. Spatial heterogeneity refers to differences in cells' location within the tumour. Understanding these various aspects of tumour heterogeneity can lead to better treatment strategies for individual patients.