An ω phase with a primitive hexagonal crystal structure has been found to be a common metastable phase in body-centered cubic (bcc) metals and alloys. In general, ω phase precipitates out as a high density of nanoscale particles and can obviously strengthen the alloys; however, coarsening of the ω particles significantly reduces the alloy ductility. The ω phase has coherent interfacial structure with its bcc matrix phase, and its lattice parameters are \( {a_{{\omega }}} = \sqrt 2 \times {a_{\text{bcc}}} \) and \( {c_{{\omega }}} = \sqrt 3 /2 \times {a_{\text{bcc}}} \). The common {112}〈111〉-type twinning in bcc metals and alloys can be treated as the product of the ω → bcc phase transition, also known as the ω-lattice mechanism. The ω phase’s behavior in metastable β-type Ti alloys will be briefly reviewed first since the ω phase was first found in the alloy system, and then the existence of the ω phase in carbon steels will be discussed. Carbon plays a crucial role in promoting the ω formation in steel, and the ω phase can form a solid solution with various carbon contents. Hence, the martensitic substructure can be treated as an α-Fe matrix embedded with a high density of nanoscale ω-Fe particles enriched with carbon. The recognition of the ω phase in steel is expected to advance the understanding of the relationship between the microstructure and mechanical properties in bcc steels, as well as the behavior of martensitic transformations, twinning formation, and martensitic substructure.