The paper examines the experimental implications of higher-dimensional theories where all standard model fields propagate in the extra dimensions, referred to as "universal extra dimensions." The authors show that the bound from electroweak data on the size of these extra dimensions is relatively loose. For a single extra dimension, the compactification scale can be as low as 300 GeV, due to the conservation of Kaluza-Klein (KK) number, which means that contributions to electroweak observables arise only from loops. The main constraint comes from weak-isospin violation effects. The direct bound on the compactification scale is set by CDF and D0 experiments in the few-hundred-GeV range. Run II of the Tevatron is expected to either discover extra dimensions or significantly raise the lower bound on the compactification scale. For two or more extra dimensions, the electroweak observables become logarithmically sensitive to the cutoff on the effective higher-dimensional theory. If the cutoff is large, the theory becomes strongly coupled and reliable predictions are not possible. If the cutoff is smaller, the observables may be estimated reliably at the one-loop level, with a lower bound on the compactification scale between 400 and 800 GeV. The paper also discusses the prospects for discovering Kaluza-Klein modes at upcoming collider experiments, including stable or long-lived KK modes and short-lived KK modes decaying promptly inside the detector. The results suggest that physics in extra dimensions may be responsible for electroweak symmetry breaking, such as the production of a bound-state Higgs doublet in six dimensions.The paper examines the experimental implications of higher-dimensional theories where all standard model fields propagate in the extra dimensions, referred to as "universal extra dimensions." The authors show that the bound from electroweak data on the size of these extra dimensions is relatively loose. For a single extra dimension, the compactification scale can be as low as 300 GeV, due to the conservation of Kaluza-Klein (KK) number, which means that contributions to electroweak observables arise only from loops. The main constraint comes from weak-isospin violation effects. The direct bound on the compactification scale is set by CDF and D0 experiments in the few-hundred-GeV range. Run II of the Tevatron is expected to either discover extra dimensions or significantly raise the lower bound on the compactification scale. For two or more extra dimensions, the electroweak observables become logarithmically sensitive to the cutoff on the effective higher-dimensional theory. If the cutoff is large, the theory becomes strongly coupled and reliable predictions are not possible. If the cutoff is smaller, the observables may be estimated reliably at the one-loop level, with a lower bound on the compactification scale between 400 and 800 GeV. The paper also discusses the prospects for discovering Kaluza-Klein modes at upcoming collider experiments, including stable or long-lived KK modes and short-lived KK modes decaying promptly inside the detector. The results suggest that physics in extra dimensions may be responsible for electroweak symmetry breaking, such as the production of a bound-state Higgs doublet in six dimensions.