This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book assesses the Court's approach to treaty interpretation. It focuses on whether the Santiago Declaration delimited the maritime boundary. The book examines the Court's finding that the parties entered into a tacit agreement. It discusses the evidence and reasoning that support this finding. The book analyses the approach to treaty interpretation by the International Court of Justice in the Maritime Dispute between Peru and Chile. The Court gave the parties almost half of what they had claimed. Chile, which claimed that the boundary had a 200 nm extension, was awarded a boundary line of 80 nm along the parallel of latitude. Peru, which claimed that the Court should apply the equidistance method to delimit the entire boundary, was given an equidistance line beyond the 80 nm point.