This book offers a comprehensive comparative overview of the effects of pre-contractual statements and agreements and post-contractual informal modifications on written contracts. It analyzes the respective case law and scholarly writing of four different jurisdictions (the US, England, Germany and Switzerland) and five uniform instruments (the CISG, PICC, PECL, DCFR and CESL) in depth, showing commonalities and differences. Topics covered are comparative contract law, modifications of (written) contracts, parol evidence rule, merger clauses, entire agreement clauses, integration clauses, no oral modification clauses, presumption of accuracy and completeness of a written contract, integrative effect of a written agreement. The book provides guidance for drafting written contracts and is of interest to practititoners and academics interested in comparative contract law.