We review the efforts on fabrication of a 3D photonic crystals by self-assembly approach employing colloidal microspheres. We will show that various defects present in these structures can give rise to significant inhomogeneous broadening of the stop-bands and eventually eliminate the photonic band gap (PBG). Here we present quantitative measurements of the higher order photonic band structure in a series of inverted opals with progressively higher refractive index contrast. Using this approach the evolution of the photonic band structure towards the opening of the complete PBG will be illustrated. The existence of the omnidirectional PBG is further explored in silicon inverted opals, which are obtained by combining planar self-assembly of colloidal spheres with silicon deposition techniques. Resulting planar, single-crystalline, high-refractive-index inverted opals of controllable thickness are integrated directly onto a silicon wafer. Using optical spectroscopy with high spatial resolution we show that defect densities in our silicon photonic crystals are sufficiently low that the PBG survives. In addition, we demonstrate that the structure can be subsequently patterned for a desired device application with straightforward post-growth processing. Thus, while retaining the simplicity of natural-assembly, this approach provides PBG crystals that reclaim the advantages of more conventional nanofabrication.