This paper considers the invalid legal transactions, which are classified according to the Civil Obligations Act as null legal transactions and voidable legal transactions. Pursuant to this classification, the reasons for nullity and voidability and their legal consequences are analysed, taking into account the deadlines for seeking legal recourse. The particularly close attention is given to the possibilities for validation and conversion of invalid legal transactions. We elaborate the concept of validation according to the Roman law, the rule that was used for validation and the cases in which the parties were able to achieve through conversion the objectives that they wished to achieve through an invalid transaction. We present the difference between the validation of null and voidable legal transactions. Regarding the validation of null legal transactions, we discuss the impossibility of validation pursuant to the Civil Obligations Act and situations in which this possibility is prescribed. We consider the validation of voidable legal transactions in view of reasons for voidability. Regarding the conversion, we establish what is necessary for the valid implementation of conversion pursuant to the provisions of the Civil Obligations Act. We explain the possibility of conversion of null legal transactions compared to the voidable legal transactions. All issues elaborated in the paper are supported by the case law of the Supreme Court of the Republic of Croatia.