shows which observables may be interesting to look at as far as searches for New Physics are concerned. We shall address both issues in detail for the particular case of $B^\pm\to\pi K$ decays, which in the context of the SM are of prime importance in determining the weak phase $\gamma=\mbox{arg}(V_{ub}^*)$ of the Cabibbo–Kobayashi–Maskawa (CKM) matrix [@NR; @me; @FM; @Robert; @mat; @Frank]. We stress, however, that trojan penguins could be important in a much wider class of processes. In particular, they may be responsible for a large contribution to the quantity $\epsilon'/\epsilon$ measuring direct CP violation in $K\to\pi\pi$ decays [@epspr; @eps1]. The importance of $B^\pm\to\pi K$ decays in the search for New Physics has been emphasized in [@me; @Mati], and some specific scenarios containing new isospin-violating contributions have been explored in [@CDK; @anom].

In Section \[sec:ops\], we discuss the effective weak Hamiltonian relevant to hadronic $B$ decays. In the presence of generic New Physics contributions, the basis of penguin operators has to be extended from the standard one in several aspects. We discuss the structure of the new operators and their scaling properties under a renormalization-group transformation from a high scale down to low energies. The theory of the rare hadronic 