This lecture notes summarize the formalism of holographic renormalization, focusing on the computation of renormalized quantum field theory (QFT) correlation functions using the AdS/CFT correspondence. The method is illustrated through the example of a massive scalar field in anti-de Sitter (AdS) spacetime. The key steps involve analyzing asymptotically AdS spacetimes, deriving the renormalized on-shell action, computing exact 1-point functions, and establishing holographic Ward identities and anomalies. The method is also applied to holographic renormalization group (RG) flows, and it is shown that results from near-boundary analysis in AdS spacetimes can be analytically continued to de Sitter (dS) spacetimes. The Brown-York stress energy tensor of dS spacetime is shown to be related to that of an associated AdS spacetime up to a sign. The discussion includes the derivation of renormalized correlation functions, RG equations, and the computation of one-, two-, and four-point functions. The method is presented in a systematic way, with emphasis on the general features and the simplest example of a massive scalar field in AdS. The results are extended to RG flows and the analysis of asymptotically dS spacetimes. The lecture notes provide a comprehensive overview of the holographic renormalization method and its applications in the context of the AdS/CFT correspondence.This lecture notes summarize the formalism of holographic renormalization, focusing on the computation of renormalized quantum field theory (QFT) correlation functions using the AdS/CFT correspondence. The method is illustrated through the example of a massive scalar field in anti-de Sitter (AdS) spacetime. The key steps involve analyzing asymptotically AdS spacetimes, deriving the renormalized on-shell action, computing exact 1-point functions, and establishing holographic Ward identities and anomalies. The method is also applied to holographic renormalization group (RG) flows, and it is shown that results from near-boundary analysis in AdS spacetimes can be analytically continued to de Sitter (dS) spacetimes. The Brown-York stress energy tensor of dS spacetime is shown to be related to that of an associated AdS spacetime up to a sign. The discussion includes the derivation of renormalized correlation functions, RG equations, and the computation of one-, two-, and four-point functions. The method is presented in a systematic way, with emphasis on the general features and the simplest example of a massive scalar field in AdS. The results are extended to RG flows and the analysis of asymptotically dS spacetimes. The lecture notes provide a comprehensive overview of the holographic renormalization method and its applications in the context of the AdS/CFT correspondence.