Hunduma LegesseMarta Ayele2025-09-122025-09-122024-09https://etd.aau.edu.et/handle/123456789/7442Motivated by a theorem of Gordon and Hedenalm (1), the study of composition operators acting on various scales of function spaces of Dirichlet series has arisen intensive interest. In this thesis, we characterize the composition operators in Hilbert space and growth estimate of the composition operator in Hilbert space of Dirichlet series.en-USGrowth EstimateComposition OperatorHilbert SpaceDirichlet SeriesThesis