An external field gives rise to a first-order transition in a smectic-A cell; the transition occurs by nucleation of focal conic domains. The physical mechanisms that govern the behavior of the new domain phase are different for small (ρ≪1) and large (ρ≫1) values of the order parameters ρ [ρ=(domain radius)/(cell thickness)]. The nucleation (ρ≪1) is governed by the elastic and field energies and is facilitated by bulk or surface irregularities, while the expansion (ρ≫1) is determined by the balance of the field and anisotropic surface energy.