In this thesis we study quantum variance for the modular surface X = Γ\H where Γ = SL 2 (Z) is the full modular group. This is an important problem in mathematical physics and number theory concerning the mass equidistribution of Maass-Hecke cusp forms on the arithmetic hyperbolic surfaces. We evaluate asymptotically the quantum variance, which is introduced by S. Zelditch and describes the fluctuations of a quantum observable. We show that the quantum variance is equal to the classical variance of the geodesic flow on S * X, the unit cotangent bundle of X, but twisted by the central value of the Maass-Hecke L-functions.Our approach is via Poincare series and Kuznetsov trace formula, which transfer the spectral sum into the sum of Kloosterman sums. The treatment of the nondiagonal terms contributions is subtle and forms the core of this thesis. It turns out that the continuous spectrum part is not negligible and contributes to the main term, but in general, it is small in the cuspidal subspace. We then make use of Watson's explicit triple product formula to determine the leading term in the asymptotic formula for the quantum variance and analyze its structure, which leads to our main result.ii Dedicated to my parents iii ACKNOWLEDGMENTS I express my deepest thanks and gratitude to my advisor, Professor Wenzhi Luo, for his patience, guidance and constant encouragement which made this work possible. I am very grateful for his enlightening discussions and suggestions as well as the intellectual support he has provided.