Companions exert adverse effects on the relationships valid for Cepheids. Physicalcompanions may also cause observable effects (e.g., phase jump in thepulsation, slight excitation of non-radial modes). Many new binaries among Cepheids can berevealed by using radial velocity measurements to be made by Gaia and by derivingastrometric 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-luminosityrelationship independently of the pulsation. A brief summary on the membership of Cepheidsin clusters is also given.