The paper presents the MSTW 2008 parton distribution functions (PDFs) at leading-order (LO), next-to-leading order (NLO), and next-to-next-to-leading order (NNLO), which supersede the previously available MRST sets. These PDFs are determined from a global analysis of hard-scattering data within the standard framework of leading-twist fixed-order collinear factorisation in the $\overline{MS}$ scheme. The new PDFs incorporate updated experimental data, including CCFR/NuTeV dimuon cross sections, Tevatron Run II data on inclusive jet production, lepton charge asymmetry from W decays, and the Z rapidity distribution. Uncertainties are propagated using a new dynamic procedure for each eigenvector of the covariance matrix. The paper discusses major changes compared to previous MRST fits, compares the results with other fitting groups, and provides predictions for W and Z total cross sections at the Tevatron and LHC. The theoretical framework includes the treatment of heavy flavours using fixed flavour number schemes (FFNS) and zero-mass variable flavour number schemes (ZM-VFNS), with a detailed discussion of the general-mass variable flavour number schemes (GM-VFNS). The paper also describes the new PDFs, their uncertainties, and the impact of the new data sets on the parton distributions. The results are compared with other PDF sets, and predictions for W and Z total cross sections are given. The paper concludes with a summary of the improvements and refinements made to the previous MRST analyses.The paper presents the MSTW 2008 parton distribution functions (PDFs) at leading-order (LO), next-to-leading order (NLO), and next-to-next-to-leading order (NNLO), which supersede the previously available MRST sets. These PDFs are determined from a global analysis of hard-scattering data within the standard framework of leading-twist fixed-order collinear factorisation in the $\overline{MS}$ scheme. The new PDFs incorporate updated experimental data, including CCFR/NuTeV dimuon cross sections, Tevatron Run II data on inclusive jet production, lepton charge asymmetry from W decays, and the Z rapidity distribution. Uncertainties are propagated using a new dynamic procedure for each eigenvector of the covariance matrix. The paper discusses major changes compared to previous MRST fits, compares the results with other fitting groups, and provides predictions for W and Z total cross sections at the Tevatron and LHC. The theoretical framework includes the treatment of heavy flavours using fixed flavour number schemes (FFNS) and zero-mass variable flavour number schemes (ZM-VFNS), with a detailed discussion of the general-mass variable flavour number schemes (GM-VFNS). The paper also describes the new PDFs, their uncertainties, and the impact of the new data sets on the parton distributions. The results are compared with other PDF sets, and predictions for W and Z total cross sections are given. The paper concludes with a summary of the improvements and refinements made to the previous MRST analyses.