In this chapter, we explain how entropy contributes to thermodynamic behavior in various systems. Entropy gives rise to several non-trivial effects, such as entropic forces in rubber, mixing phenomena in multicomponent systems, and degeneracy-induced phenomena (e.g., spin-crossover systems) and so on. Phase transitions can also be driven by entropy effects. Entropy plays a crucial role in systems with competing interactions (frustration), which will be discussed in the next part.