Energy cascades is one of the defining features of 3D turbulence, however their mathematical theory remains elusive. In this talk we will describe how energy cascades can be studied using the incompressible Navier-Stokes equations. In particular, we will discuss the phenomenon of dissipation anomaly where energy cascades are triggered in the vanishing viscosity limit towards a solution of Euler equations that possesses singularities which dissipate energy (anomalous dissipation). In 1949, Onsager made a conjecture about existence of such solutions as well as their minimal regularity (smoothness). The mathematical problem behind Onsager Conjecture attracted considerable attention with a number of recent breakthroughs.