The main aim of this chapter is to reconstruct the genesis and significance of Jonas’s philosophy of life, which corresponds to the second phase of his philosophical production (from the mid 1940s). Jonas’s philosophy of life will be analysed in the light of published sources, as well as less-known, un-published or recently published ones, and contextualised as regards the overall itinerary of his thinking and with respect to past and present philosophical and scientific debates, including those concerning the knowability of life by science, the meaning of evolution, and the anthropological difference. The theoretical hypothesis that will be argued is that, despite its gaps and discontinuities, Hans Jonas’s philosophy of life is not only intimately coherent, but also an organic expression of the author’s thinking and, therefore, capable of shedding light on the unity underlying its multifaceted and engaged character.