This chapter conceptualises solidarity between the member states on the basis of two spectrums. The first deals with the reasons for acting in the interest of the collective. Its poles are taken up by normative and factual solidarity. Normative solidarity occurs when states display solidarity due to a political obligation. The chapter explains that such political obligations are grounded in joint commitments, how these commitments turn their participants into a unity, and that they may not only exist between individuals, but also between states. Factual solidarity occurs when a state acts in the interest of the whole because it serves its own interest. Two situations are singled out: interdependence and common destiny. The second spectrum is subsidiary in nature and relates to the kind of solidary behaviour displayed. Its poles are negative and positive solidarity. Negative solidarity occurs when the acts displayed by a state in support of the whole relate to itself. Positive solidarity occurs when a state acts in the interest of the collective by directly benefitting another state. The chapter concludes with a discussion of the relation between joint commitments and Union law.