Zero-phonon absorption and fluorescence lines from single molecules can be tuned by applying an external electric field a phenomenon known as the Stark effect. The majority of single-molecule systems possess the center of symmetry and in this case we observe a quadratic frequency shift of the zero-phonon lines. A well-known example are dibenzoterrylene (DBT) molecules in an anthracene crystal. A linear Stark effect, visualized as the linear frequency shift of the lines following the electric field, is associated to a lack of centrosymmetry of the guest molecule in the host matrix, which creates the change of its electric dipole moment. DBT in 2,3-dibromonaphthalene crystal is the first successful demonstration of a strategy to use an asymmetric and polar host matrix to induce the large static dipole moments in otherwise centrosymmetric guest molecule. The Stark-shifts of the molecular electronic levels can be also induced optically by focusing an intense laser beam on the sample. A photoionization cascade is proposed in order to explain the creation of the local and stable electric field in a crystalline matrix.