Flat bands are single-particle energy bands in tight-binding networks that exhibit macroscopic degeneracy and extreme sensitivity to perturbations. They are promising candidates for exotic phases and unconventional orders. Constructing flat band networks requires symmetries and fine-tuning. This review discusses recent systematic methods for constructing flat band networks based on symmetries or fine-tuning, and how these can be extended, adapted, or exploited in the presence of perturbations. These methods have led to discoveries such as non-perturbative metal-insulator transitions, fractal phases, nonlinear and quantum caging, and many-body nonergodic quantum models. The implications of these results for the design of fine-tuned nanophotonic systems, including photonic crystals, nanocavities, and metasurfaces, are discussed. Flat bands are dispersionless bands in wave structures that can propagate through a spatially periodic medium. They are often accompanied by compact localized eigenstates (CLS). The 2D Lieb lattice is an example, with CLS occupying four sites and a band structure with two dispersive and one flat band. Photonic realizations of CLS and flat bands have been demonstrated using femtosecond-laser writing on silica glass. Flat bands result in macroscopic degeneracy and absence of transport, which can be broken by perturbations, leading to novel transporting phases. Perturbations become the key to switching between different phases of matter. Fine-tuning is essential to control these perturbations. Flat band lattices themselves are part of manifolds and respond differently to the same perturbation. Fine-tuning within these manifolds is a desirable scheme for switching between phases. Flat band physics has evolved through several cycles. Early work by Sutherland showed that CLS are not unique to quasicrystal tight-binding models but persist in systems with strict discrete translational invariance. Lieb used the Lieb lattice for superconductivity and introduced the concept of chiral symmetry protected flat bands. Mielke and Tasaki contributed to the first evolution cycle by developing flat band generators and showing that flat bands can be preserved under fine-tuned perturbations. The second cycle saw significant theoretical progress, with many aspects of flat band properties identified. However, systematically searching for new flat band models remained challenging due to the limitations of condensed matter material science. The third cycle focused on systematic and complete flat band generators, using CLS as a starting point rather than an outcome. This led to the first and complete flat band generator for one-dimensional two-band lattices. Flat bands can be classified into different types based on the properties of their CLS sets. Orthogonal flat bands have complete and orthonormal CLS sets, while linearly independent and dependent flat bands have different characteristics. Flat bands can be further categorized into ABF (all bands flat) lattices, which have all bands flat and no transport. FineFlat bands are single-particle energy bands in tight-binding networks that exhibit macroscopic degeneracy and extreme sensitivity to perturbations. They are promising candidates for exotic phases and unconventional orders. Constructing flat band networks requires symmetries and fine-tuning. This review discusses recent systematic methods for constructing flat band networks based on symmetries or fine-tuning, and how these can be extended, adapted, or exploited in the presence of perturbations. These methods have led to discoveries such as non-perturbative metal-insulator transitions, fractal phases, nonlinear and quantum caging, and many-body nonergodic quantum models. The implications of these results for the design of fine-tuned nanophotonic systems, including photonic crystals, nanocavities, and metasurfaces, are discussed. Flat bands are dispersionless bands in wave structures that can propagate through a spatially periodic medium. They are often accompanied by compact localized eigenstates (CLS). The 2D Lieb lattice is an example, with CLS occupying four sites and a band structure with two dispersive and one flat band. Photonic realizations of CLS and flat bands have been demonstrated using femtosecond-laser writing on silica glass. Flat bands result in macroscopic degeneracy and absence of transport, which can be broken by perturbations, leading to novel transporting phases. Perturbations become the key to switching between different phases of matter. Fine-tuning is essential to control these perturbations. Flat band lattices themselves are part of manifolds and respond differently to the same perturbation. Fine-tuning within these manifolds is a desirable scheme for switching between phases. Flat band physics has evolved through several cycles. Early work by Sutherland showed that CLS are not unique to quasicrystal tight-binding models but persist in systems with strict discrete translational invariance. Lieb used the Lieb lattice for superconductivity and introduced the concept of chiral symmetry protected flat bands. Mielke and Tasaki contributed to the first evolution cycle by developing flat band generators and showing that flat bands can be preserved under fine-tuned perturbations. The second cycle saw significant theoretical progress, with many aspects of flat band properties identified. However, systematically searching for new flat band models remained challenging due to the limitations of condensed matter material science. The third cycle focused on systematic and complete flat band generators, using CLS as a starting point rather than an outcome. This led to the first and complete flat band generator for one-dimensional two-band lattices. Flat bands can be classified into different types based on the properties of their CLS sets. Orthogonal flat bands have complete and orthonormal CLS sets, while linearly independent and dependent flat bands have different characteristics. Flat bands can be further categorized into ABF (all bands flat) lattices, which have all bands flat and no transport. Fine