Companions exert adverse effects on the relationships valid for Cepheids. Physical companions may also cause observable effects (e.g., phase jump in the pulsation, slight excitation of non-radial modes). Many new binaries among Cepheids can be revealed by using radial velocity measurements to be made by Gaia and by deriving astrometric orbit from data to be obtained over the five years of the Gaia mission. Cluster membership of Cepheids also facilitates the calibration of the period-luminosity relationship independently of the pulsation. A brief summary on the membership of Cepheids in clusters is also given.