We define a C * -hull for a * -algebra, given a notion of integrability for its representations on Hilbert modules. We establish a local-global principle which, in many cases, characterises integrable representations on Hilbert modules through the integrable representations on Hilbert spaces. The induction theorem constructs a C * -hull for a certain class of integrable representations of a graded * -algebra, given a C * -hull for its unit fibre. Contents 1. Introduction 1 2. Representations by unbounded operators on Hilbert modules 5 3. Integrable representations and C * -hulls 10 4. Polynomials in one variable I 15 5. Local-Global principles 19 6. Polynomials in one variable II 27 7. Bounded and locally bounded representations 30 8. Commutative C * -hulls 37 9. From graded * -1--------algebras to Fell bundles 43 10. Locally bounded unit fibre representations 53 11. Fell bundles with commutative unit fibre 57 12. Rieffel deformation 64 13. Twisted Weyl algebras 66 References 71 2010 Mathematics Subject Classification. Primary 47L60; Secondary 46L55. Key words and phrases. unbounded operator; regular Hilbert module operator; integrable representation; induction of representations; graded * -algebra; Fell bundle; C * -algebra generated by unbounded operators; C * -envelope; C * -hull; host algebra; Weyl algebra; canonical commutation relations; Local-Global Principle; Rieffel deformation.