We deal with the conditional regularity of the weak solutions to the Navier–Stokes equations. We discuss a famous criterion by Vasseur in terms of
divfalse(bold-italicufalse/false|bold-italicufalse|false) and extend this criterion to bounded domains with Navier and Navier‐type boundary conditions. Inspired by the equality
bold-italicu·∇false|bold-italicufalse|λ=−λfalse|bold-italicufalse|λ+1divfalse(bold-italicufalse/false|bold-italicufalse|false),0.1emλ≥1, we further prove an optimal regularity criterion in terms of
bold-italicu·∇false|bold-italicufalse|λ both for the whole three‐dimensional space and bounded domains with Navier's, Navier‐type, and Dirichlet boundary conditions. It specially means for
λ=2 that the control of the energy flow in the critical norms
LtpLxr provides the regularity of solutions. This criterion is proved by two different techniques.