The results of a comprehensive analysis of existing data on the weak neutral current and the W and Z masses are presented. The principal results are the following. (a) There is no evidence for any deviation from the standard model. (b) A global fit to all data yields ${\mathrm{sin}}^{2}$${\mathrm{\ensuremath{\theta}}}_{\mathrm{W}}$\ensuremath{\equiv}1-${\mathrm{M}}_{\mathrm{W}}$${\mathrm{}}^{2}$/${\mathrm{M}}_{\mathrm{Z}}$${\mathrm{}}^{2}$=0.230\ifmmode\pm\else\textpm\fi{}0.0048, where this error and all others given here include full statistical, systematic, and theoretical uncertainties (computed assuming three fermion families, ${m}_{t}$\ensuremath{\le}100 GeV, and ${M}_{H}$\ensuremath{\le}1 TeV). (c) Allowing \ensuremath{\rho}\ensuremath{\equiv}${M}_{W}$${\mathrm{}}^{2}$/(${M}_{Z}$${}^{2}$${\mathrm{cos}}^{2}$${\mathrm{\ensuremath{\theta}}}_{\mathrm{W}}$) as well as ${\mathrm{sin}}^{2}$${\mathrm{\ensuremath{\theta}}}_{\mathrm{W}}$ to vary one obtains ${\mathrm{sin}}^{2}$${\mathrm{\ensuremath{\theta}}}_{\mathrm{W}}$=0.229\ifmmode\pm\else\textpm\fi{}0.0064 and \ensuremath{\rho}=0.998\ifmmode\pm\else\textpm\fi{}0.0086. This implies 90%-confidence-level (C.L.) upper limits of 0.047 and 0.081 for the vacuum expectation values (relative to those of Higgs doublets) for Higgs triplets with weak hypercharge of 0 and \ifmmode\pm\else\textpm\fi{}1, respectively. (d) The parameter ${\ensuremath{\delta}}_{W}$\ensuremath{\equiv}\ensuremath{\Delta}r-\ensuremath{\Delta}${s}^{2}$(1-\ensuremath{\Delta}r)/${\mathrm{sin}}^{2}$${\mathrm{\ensuremath{\theta}}}^{0}$, which is a measure of the radiative corrections relating deep-inelastic neutrino scattering, the W and Z masses, and muon decay, is determined to be 0.112\ifmmode\pm\else\textpm\fi{}0.037. This is consistent with the value ${\ensuremath{\delta}}_{W}$=0.106 expected for ${m}_{t}$=45 GeV and ${M}_{H}$=100 GeV and establishes the existence of radiative corrections at the 3\ensuremath{\sigma} level. (e) The radiative corrections are sensitive to isospin breaking associated with a large ${m}_{t}$.Assuming no deviation from the standard model, consistency of the various reactions requires ${m}_{t}$180 GeV at 90% C.L. for ${M}_{H}$\ensuremath{\le}100 GeV, with a slightly weaker limit for larger ${M}_{H}$. Similar results hold for the mass splittings between fourth-generation quarks or leptons. (f) Most of the parameters in model-independent fits to \ensuremath{\nu}q, \ensuremath{\nu}e, eq, and ${e}^{+}$${e}^{\mathrm{\ensuremath{-}}}$ processes are now determined uniquely and precisely. (g) Limits are given on the masses and mixing angles of additional Z bosons expected in popular models. For theoretically expected coupling constants one finds that the neutral-current constraints are usually more stringent than the direct-production limits from the CERN Sp\ifmmode\bar\else\textasciimacron\fi{}pS collider, but nevertheless masses as low as 120--300 GeV are typically allowed. (h) The implications of these results for grand unification are discussed. ${\mathrm{sin}}^{2}$${\mathrm{\ensuremath{\theta}}}_{\mathrm{W}}$ is \ensuremath{\ge}2.5 standard deviations above the prediction of minimal SU(5) and similar models for all ${m}_{t}$. It is closer to the prediction of simple supersymmetric grand unified theories but is still somewhat low. (i) The dominant theoretical uncertainty (the charm-quark threshold in deep-inelastic charged-current scattering) is considered in some detail.