Canine histiocytic sarcoma (HS) is a highly aggressive tumour of histiocytic origin and it accounts for up to 50% of malignant tumours in flatcoated retrievers (FCR). Effective treatment has yet to be found, and prognosis is always poor. In recent years, in human and veterinary oncology, there has been a growing interest in regulatory T cells (Treg). This population of cells has the ability to down-regulate the immune system, aiding tumour growth and diffusion. In the literature evidence can be found of Treg infiltrating tumours and of elevation of percentages of Treg in peripheral blood of dogs and people with cancer. The aims of this project were to evaluate the presence of Treg infiltrating canine HS and to interrogate the tumour microenvironment to assess the molecules involved in tumour immuneevasion. Peripheral blood was analysed to determine the percentage of Treg in dogs bearing HS and age-matched controls. In this study 27 archive samples of HS were immunolabelled with MHC class II, E-cadherin, IL-10, CD3, FoxP3 and PD-L1 (26 samples). Eight blood samples from dogs bearing HS were analysed via flow cytometry to identify percentages of T cell (CD3+CD4+ and CD3+CD8+) and of Treg (CD4+, CD25+, FoxP3+). Findings showed no significant difference in the percentage of Treg in peripheral blood of disease FCR. However, a trend was found in a decrease of CD3+ cells in dogs with HS, driven by a reduction of CD3+CD8+ cells. Within the affected group a higher proportion of CD3+CD4- CD8- cells (possibly $\gamma$$\delta$ T cells) was also observed. Immunohistochemistry identified strong staining for MHC class II in all samples, confirming the presence of antigen presenting cells; and no staining for E-cadherin suggested mature dendritic cells as the cells of origin. Infiltrating cells stained strongly for CD3 and between 1% and 20% of these cells expressed FoxP3, confirming the presence of Treg. Surprisingly, low staining for IL-10 suggests that the release of this cytokine by Treg plays a minor role in immune evasion in canine HS. Antibody validation for a cross-reactive anti-PD-L1 antibody was achieved through immunofluorescence analysis of frozen HS samples and assessment of mRNA expression though PD-1 and PD-L1 assays on RT-qPCR. On FFPE samples positivity for PD-L1 was found in all samples, and overall 41% to 60% of neoplastic cells expressed the marker. This project has confirmed infiltration of Treg within the microenvironment of canine HS and has identified a possible role for the PD-1/PD-L1 pathway in canine HS immune evasion.