I. Hodge structures attached to primitive forms of weight 2.- 0. Definitions and notations.- 1. Hodge structures attached to Hilbert modular surfaces.- 2. Hodge structures attached to primitive forms of weight 2.- 3. Nonholomorphic involutive automorphisms of Hilbert modular surfaces.- 4. Period relation of Riemann-Hodge.- II. Abelian varieties attached to primitive forms.- 5. Abelian varieties attached to Clifford algebras.- 6. The period moduli of the isogeny classes Atf ? Q and A2f ? Q.- 7. Main theorem A and its corollaries.- 8. Selfconjugate forms and algebraic cycles.- 9. Abelian varieties attached to non-selfconjugate forms.- III. Correspondence between real Nebentype elliptic modular forms and Hilbert modular forms.- 10. Weil representation and theta series.- 11. Construction of real Nebentype elliptic modular forms.- 12. Construction of Hilbert modular forms.- 13. The adjointness formula.- IV. Period relation for the lifting of modular forms and transcendental cycles.- 14. Hodge structures attached to real Nebentype elliptic modular forms of weight 2.- 15. Construction of 2-cycles.- 16. Arithmetic index theorems.- 17. Period relation for the Doi-Naganuma lifting and Main Theorem B.- 18. Selfconjugate forms and transcendental cycles.- 19. Notes on ?-adic cohomology groups of certain Hilbert modular surfaces.- 20. Remarks.- Bibliography..