We have critically reviewed and discussed currently available information regarding the spin and valence states of iron in lower mantle minerals and the associated effects of the spin transitions on physical, chemical, and transport properties of the deep Earth. A high‐spin to low‐spin crossover of Fe2+ in ferropericlase has been observed to occur at pressure‐temperature conditions corresponding to the middle part of the lower mantle. In contrast, recent studies consistently show that Fe2+ predominantly exhibits extremely high quadrupole splitting values in the pseudo‐dodecahedral site (A site) of perovskite and post‐perovskite, indicative of a strong lattice distortion. Fe3+ in the A site of these structures likely remains in the high‐spin state, while a high‐spin to low‐spin transition of Fe3+ in the octahedral site of perovskite occurs at pressures of 15–50 GPa. In post‐perovskite, the octahedral‐site Fe3+ remains in the low‐spin state at the pressure conditions of the lowermost mantle. These changes in the spin and valence states of iron as a function of pressure and temperature have been reported to affect physical, chemical, rheological, and transport properties of the lower mantle minerals. The spin crossover of Fe2+ in ferropericlase has been documented to affect these properties and is discussed in depth here, whereas the effects of the spin transition of iron in perovskite and post‐perovskite are much more complex and remain debated. The consequences of the transitions are evaluated in terms of their implications to deep Earth geophysics, geochemistry, and geodynamics including elasticity, element partitioning, fractionation and diffusion, and rheological and transport properties. Key Points Spin states of iron in lower‐mantle minerals are reviewed. Effects of the transitions are addressed. Implications of the transitions are discussed.