Nearly ten percent of patients with colorectal cancer will develop spread of the cancer to the membrane lining their abdomen, called the peritoneum. The cells which make up this peritoneum, mesothelial cells, have been shown to be able to influence cancer progression. Cancer cells can communicate with many types of cells, and influence healthy cells to help cancer cells grow. This leads to the presence of several types of normally healthy cells in a tumour. In the peritoneum, cancer cells influence mesothelial cells to make the peritoneum, which normally defends against cancer spreading, into an environment where cancer cells can grow. This process has been studied in gastric and ovarian cancer, but not yet in colorectal cancer. We will utilize new techniques to accurately define what types of cells are present in tumours which have spread to the peritoneum. With this new technique, we are able to see what genes are active in each type of cell in these tumours. This way, we can determine where cells which are present in tumours come from, and what their functions are. We will perform experiments to find out how cancer cells communicate with mesothelial cells, and how mesothelial cells help colorectal cancer grow. Later on, we will try to disrupt this communication and see if this strategy might be useful to develop new treatments for treating or preventing colorectal cancer spreading to the peritoneum.