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.