Current observational estimates suggest that ~30% of all binary stars are in triple systems. In ~70% of these, the enclosed binary is close enough that the primary in the binary can evolve to fill its Roche lobe. In ~10%-20%, the third, more distant component can evolve to fill its Roche lobe, leading to configurations inaccessible to isolated binary stars. Triple stars are unstable if the ratio of the orbital period of the enclosed binary to the period of the third component exceeds a critical value. Hence, an increase in the orbital period of the binary due to conservative mass transfer between components or to wind mass loss from the binary can destabilize an initially stable triple system, causing it to decompose into a rapidly moving single star and an evolved binary recoiling in the opposite direction with a velocity large compared with velocities typical of primordial binaries. To highlight the different possibilities inherent in triple-star evolution, we discuss qualitatively several possible scenarios whereby triple stars with component masses in the range 1-10 M☉ can evolve into Type Ia supernovae, which we assume to be explosions of merging carbon-oxygen or oxygen-neon white dwarfs of total mass larger than 1.4 M☉. Before quantitative predictions of the likelihood of these scenarios can be made, it is necessary to determine the initial distribution of young triple stars over their masses and orbital separations and to calculate the reaction of the enclosed binary to matter transferred to it by the third component when it fills its Roche lobe or supports a strong wind.
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