  We review the construction of homological evolutionary vector fields on
infinite jet spaces and partial differential equations. We describe the
applications of this concept in three tightly inter-related domains: the
variational Poisson formalism (e.g., for equations of Korteweg-de Vries type),
geometry of Liouville-type hyperbolic systems (including the 2D Toda chains),
and Euler-Lagrange gauge theories (such as the Yang-Mills theories, gravity, or
the Poisson sigma-models). Also, we formulate several open problems.
