The paper discusses the bounds on universal extra dimensions (UED) accessible to all standard model (SM) fields. It shows that the bound on the compactification scale from electroweak data is relatively loose, allowing a compactification scale as low as 300 GeV for one extra dimension. This is because the Kaluza-Klein (KK) number is conserved, leading to contributions to electroweak observables only from loops. The main constraint comes from weak-isospin violation effects. The paper computes contributions to the S parameter and the Zb̄b vertex. The direct bound on the compactification scale is set by CDF and D0 in the few hundred GeV range, and the Tevatron Run II will either discover extra dimensions or significantly raise the bound. For two UED, the current lower bound on the compactification scale depends logarithmically on the ultraviolet cutoff, but can be estimated between 400 and 800 GeV. With three or more extra dimensions, the cutoff dependence is too strong to allow an estimate. The paper discusses the Kaluza-Klein spectrum and interactions in higher-dimensional theories. It shows that the momentum conservation in the extra dimensions is preserved by the orbifold projection. The KK modes contribute at loop-level to the electroweak observables, allowing some lower bound on the compactification scale. The paper also discusses the electroweak data versus extra dimensions, showing that the T and S parameters are sensitive to the higher-dimensional physics. The paper concludes that the current lower bound on the compactification scale is set by the direct production experiments, and that the Tevatron Run II will either discover extra dimensions or significantly raise the bound. The paper also discusses the prospects for discovering Kaluza-Klein modes, noting that the direct lower bound on 1/R competes with or even exceeds the indirect bound set by the electroweak precision measurements. The paper concludes that Run II at the Tevatron will either discover a universal extra dimension or else it will significantly increase the lower bound on the compactification scale.The paper discusses the bounds on universal extra dimensions (UED) accessible to all standard model (SM) fields. It shows that the bound on the compactification scale from electroweak data is relatively loose, allowing a compactification scale as low as 300 GeV for one extra dimension. This is because the Kaluza-Klein (KK) number is conserved, leading to contributions to electroweak observables only from loops. The main constraint comes from weak-isospin violation effects. The paper computes contributions to the S parameter and the Zb̄b vertex. The direct bound on the compactification scale is set by CDF and D0 in the few hundred GeV range, and the Tevatron Run II will either discover extra dimensions or significantly raise the bound. For two UED, the current lower bound on the compactification scale depends logarithmically on the ultraviolet cutoff, but can be estimated between 400 and 800 GeV. With three or more extra dimensions, the cutoff dependence is too strong to allow an estimate. The paper discusses the Kaluza-Klein spectrum and interactions in higher-dimensional theories. It shows that the momentum conservation in the extra dimensions is preserved by the orbifold projection. The KK modes contribute at loop-level to the electroweak observables, allowing some lower bound on the compactification scale. The paper also discusses the electroweak data versus extra dimensions, showing that the T and S parameters are sensitive to the higher-dimensional physics. The paper concludes that the current lower bound on the compactification scale is set by the direct production experiments, and that the Tevatron Run II will either discover extra dimensions or significantly raise the bound. The paper also discusses the prospects for discovering Kaluza-Klein modes, noting that the direct lower bound on 1/R competes with or even exceeds the indirect bound set by the electroweak precision measurements. The paper concludes that Run II at the Tevatron will either discover a universal extra dimension or else it will significantly increase the lower bound on the compactification scale.