In this chapter, we study the ground states and ordering processes of frustrated systems with discrete variables, such as Ising spins ( \(\sigma _i = \pm 1\) ): 4.1 \(\begin{aligned} \mathcal{H}=-\sum _{\langle ij \rangle }J_{ij}\sigma _i\sigma _j, \end{aligned}\) where \(\langle ij \rangle \) denotes a pair of sites interacting with the coupling constant \(J_{ij}\) . Here, \(J_{ij}\) can be positive for ferromagnetic couplings or negative for antiferromagnetic couplings, and not all interactions can be satisfied simultaneously. These models exhibit macroscopically degenerate ground states, resulting in residual entropy at \(T=0\) . The nature of the degenerate ground state may lead to an order-by-disorder type phase transition. In frustrated systems, the ordering process is largely suppressed, and the nature of the phase transition is significantly modified, resulting in peculiar phenomena, such as partially disordered phases, reentrant phase transitions, and so on.