Osteoarthritis (OA) is the most common degenerative joint disease affecting more than 40% of people above the age of 65 (Neogi et al., 2013). Obesity is one of the main risk factors of OA and has become a major problem in Western societies. With sedentary lifestyle and the aging of the population, it is estimated that more than 50% of British adults will be obese in 2030 (Wang et al., 2011). So far, the effect of obesity on joint degeneration has primarily been explained by the increased load on the joints. However, a growing number of studies have revealed that adipose tissue can affect cartilage and other joint tissues at a molecular level. The main goal of this thesis was to investigate the role of local knee joint tissues in obese patients with OA. The expression of molecular markers was investigated in local knee tissues: cartilage, synovium, infrapatellar fat pad (IPFP) and subchondral bone collected during Total Knee Replacement (TKR). A range of techniques (RT-PCR, Real Time qPCR, WB, IHC/ICC and ELISA) was used to examine the differences between genes and proteins expression in both lean and obese patients with OA. Further, the local immune cell infiltration was investigated in knee adipose tissue depots (synovium and IPFP) using flow cytometry. In addition, the subchondral bone microstructure was analysed using micro-Computed Tomography (μCT) and IHC techniques. Chondrocytes from OA patients were found to express a range of obesity-related genes. ADIPOR1 was produced significantly higher than ADIPOR2 in OA chondrocytes. Furthermore, CCL2 was produced at higher while PPARγ and visfatin were produced at a lower level in obese patients’ chondrocytes in comparison to lean ones. Synovium and IPFP also expressed a range of obesity-related genes. PPARγ and visfatin expression was lower in obese synovium and IPFP in comparison to lean. Surprisingly, adiponectin was expressed at a significantly lower level in obese patients’ synovium. In contrast, adiponectin was not differently expressed in lean and obese patients’ IPFP. The IPFP was found to be a significantly higher producer of PPARγ and adiponectin in comparison to synovium. Synovium, on the other hand, has an increased expression of VCAM-1, TLR4 and CCL2 in obese patients. An increased number of macrophages (defined by CD45+CD14+ and CD14+CD206+ markers expression) was detected in the synovium and IPFP from obese OA patients. Furthermore, there was an increased number of CD86+CD14+ cells in the synovium from obese patients. Other macrophage-related proteins including HLA-DR, CD36 were also expressed at a higher level in synovium from obese patients. T-lymphocyte detection revealed a higher number of CD3+CD4+ T cells in the synovium (but not IPFP) from obese patients but no change in the CD3+CD8+ population in both the synovium and IPFP. Subchondral bone analysis revealed possible differences in this tissue in obese male patients with OA in comparison to lean patients. μCT examination of subchondral bone showed a significantly lower bone mineral density (BMD) in obese in comparison to lean male OA patients. IHC analysis of bone sections suggested that there was an increased number of bone marrow adipose tissue macrophages. In addition, osteoblasts obtained from obese OA donors expressed a significantly higher level of ADIPOR2 and lower level of PPARγ mRNA in comparison to lean patients’ osteoblasts. The data obtained suggests that there were differences between lean and obese patients with OA at a molecular level. This proposes possible future directions for targeting these diseases. The limitation of the study were as follows: 1) possible different stages of end-stage OA between analysed patients, which could lead to differences in obtained data, 2) no non-OA control samples included in the study. However, the presented study may suggest that all tissues in the knee joint contribute to the interplay between OA and obesity. In addition, the data obtained is the first to suggest that there are differences in gene and protein expression in the synovium and IPFP from the same donor. Furthermore, there are differences in the immune cell populations in local adipose tissue depots (synovium and IPFP) from OA joints, which are linked to obesity. All of this data has helped to increase our understanding of the interaction between obesity and OA.