The ius testandi over a peculium castrense granted to filii familias milites opened the way towards an increasing independence of children-in-power with regard to property rights. This book especially focusses on the foundations of the institution and investigates its dark ages. It covers the legal history of the peculium castrense in a strictly chronological manner and follows the evolution of this special peculium during the periods from Augustus to Hadrian. The effect of granting the filius familias miles the ability to testate as well as the changes resulting from his integration into the structure and organisation of the army are investigated in relation to the patria potestas. A further topic discussed in depth are what may have been the motifs of a legal policy that established an ius testandi of filii familias milites and (since Hadrian) also of veterans still under the power of their fathers.