This text was originally published in Portuguese by Síntese (Deus, 2019). This chapter addresses the influence of Karl Marx’s empirical research related to the crises of 1857 and 1866 on the process of writing Capital and the impact of this process on Marx’s own analysis of economic crises. It is a theoretical reflection on Marx’s analysis of the crises of 1857 and 1866, drawing from both published and unpublished works. While the first crisis was extensively examined and commented in the first and third volumes of Capital, the second crisis was thoroughly investigated by Marx after the initial publishing of the first volume of Capital in 1867. However, these findings were not included in Volume III: it would have been challenging for Engels to incorporate those excerpts and notes into his editorial work of the 1880s and 1890s. Nonetheless, the lessons that can be gleaned from these sources, now available through MEGA2, not only provide empirical evidence of the incompleteness of Marx’s seminal work but also have the potential to inform new interpretations of Capital.