Wilson's RG: integrate out high-momentum modes (Λ/b < |k| < Λ), then rescale. The theory "flows" in the space of couplings.
Fixed points: Gaussian (free theory, r*=u*=0) is UV fixed point. Wilson-Fisher fixed point (ε-expansion: u* = ε/6 + O(ε²)) controls the 3D Ising universality class.
Relevant operators (grow under RG) correspond to fine-tuning; irrelevant operators (shrink) explain universality — different microscopic theories flow to the same fixed point.
The critical exponent ν = 1/y_t where y_t = 2 − η is the thermal RG eigenvalue. At d=4−ε: ν = 1/2 + ε/12 + O(ε²).