Property is one of the fundaments of ancient, medieval and modern society; the relevant legislation is of eminent importance and is still dominated by principles of Roman law: – The so-called abstraction principle is applied Roman law: It separates the contractual agreement and the acquistion of property. If the contractual agreement is invalid, there is given a so-called >condictio indebiti<, even if the translation of property is valid. – The acquisition of property in good faith is based on Roman law; the prinicple of acquisition in good faith protects the legal relations and postpones the interest of the (former) owner. – The relationships between the property owner and the unrightful possessor may be rather intricated, Roman law ...