This work of translation of the classic Russian text book emphasizes a fresh modern approach to the geometric qualitative theory of ordinary differential equations. The subject matter of this book is dominated by two central ideas and their ramifications: the theorem on rectifiability of a vector field and the theory of one-parameter groups of linear transformations. While the author has taken the liberty to omit some of the more specialized topics usually included in books on ordinary differential equations, the applications of these equations to mechanics, on the contrary, have been considered in more than the customary detail.