Military lessons of the war in Ukraine are numerous and of great interest to Western armies. They affect all areas of conflict and are diverse and varied. Against this backdrop, this article looks in particular at the use of reservists by the two belligerents and seeks to draw some useful observations for our own defence models. Until the mid 2000s, the Russian and Ukrainian armed forces, both heirs to the Soviet model, had similar structures. Then these structures, and in particular the organisation, training and use of reservists diverged. The conflict that began in 2014 has accentuated these divergences, challenged the models chosen but also shown the importance of using reservists. Since 2014, but especially since 2022, both the Russians and the Ukrainians have had to adapt the way they employ their reservists in order to face the reality of war, with contrasting results. For their part, Western armies have been grappling since the end of the Cold War with the dilemma: mass or high technology? The question of this use of reservists in France and more widely in Western countries is no exception to this issue. First, this study describes the models in place and how they are evolving, then it analyses the use and results obtained by Ukrainian and Russian reservists in the context of the current conflict. Finally, in the light of these observations, this document proposes three main options for Western armies regarding the organisation and the use of their reservists.