As we have studied in previous chapters, various configurations are energetically degenerate or nearly degenerate in frustrated systems. This leads to a rich variety of phases. In such systems, thermal fluctuations (entropy effects) play an important role in selecting the thermodynamically stable state, a phenomenon known as “order-by-disorder”order-by-disorder. At zero temperature, quantum fluctuations play a crucial role in determining the ground state, and quantum phase transitions can occur as a result. Quantum fluctuations in spin systems originate from the non-commutativity of spin operators. These fluctuations reduce the magnitude of the classical order parameter defined by spin orientation, while lowering the ground-state energy. These behaviors are quite counterintuitive but are essential for understanding the ground states of frustrated quantum systems. Degeneracy in classical ground states may be resolved quantum mechanically; that is, some linear combination of the degenerate classical states can become the true ground state. Moreover, even if a state’s classical energy is higher, the energy gain due to quantum fluctuations can lower its total energy below that of the classical ground state. Such a reduction in ground-state energy due to quantum fluctuations gives rise to an intriguing phenomenon known as “order-by-quantum fluctuation”order-by-quantum fluctuation  (Nikuni and Shiba in J Phys Soc Jpn 62:3268 (1993) [1], Chubukov in Phys Rev Lett 69:832 (1992) [2]) . In this chapter, we briefly overview quantum effects on ordered states and review unusual quantum phase transitions in frustrated quantum systems.