The paper presents updated parton distribution functions (PDFs) at leading-order (LO), next-to-leading-order (NLO), and next-to-next-to-leading-order (NNLO) for the Large Hadron Collider (LHC) era. These PDFs, referred to as "MSTW 2008," are derived from a global analysis of hard-scattering data within the standard framework of leading-twist fixed-order collinear factorization in the $\overline{\text{MS}}$ scheme. The new PDFs supersede the previously available "MRST" sets and are designed for the first LHC data-taking and associated theoretical calculations. The analysis includes new data sets such as CCFR/NuTeV dimuon cross sections, which constrain strange quark and antiquark distributions, and Tevatron Run II data on inclusive jet production, lepton charge asymmetry from $W$ decays, and $Z$ rapidity distribution. The uncertainties are propagated from experimental errors using a dynamic procedure for each eigenvector of the covariance matrix. The paper discusses major changes compared to previous MRST fits, compares with other PDF sets, and provides predictions for $W$ and $Z$ total cross sections at the Tevatron and LHC.The paper presents updated parton distribution functions (PDFs) at leading-order (LO), next-to-leading-order (NLO), and next-to-next-to-leading-order (NNLO) for the Large Hadron Collider (LHC) era. These PDFs, referred to as "MSTW 2008," are derived from a global analysis of hard-scattering data within the standard framework of leading-twist fixed-order collinear factorization in the $\overline{\text{MS}}$ scheme. The new PDFs supersede the previously available "MRST" sets and are designed for the first LHC data-taking and associated theoretical calculations. The analysis includes new data sets such as CCFR/NuTeV dimuon cross sections, which constrain strange quark and antiquark distributions, and Tevatron Run II data on inclusive jet production, lepton charge asymmetry from $W$ decays, and $Z$ rapidity distribution. The uncertainties are propagated from experimental errors using a dynamic procedure for each eigenvector of the covariance matrix. The paper discusses major changes compared to previous MRST fits, compares with other PDF sets, and provides predictions for $W$ and $Z$ total cross sections at the Tevatron and LHC.