In recent years, photonic integration has attracted much attention owing to its revolutionary potentials in next-generation computing architecture. In this talk, the challenges and opportunities of applying topological concepts to on-chip photonic integration are review. In particular, we focus on a platform of photonic crystals. It is found that, the bound states in the continuum in such a system are essentially topological defects: the integer windings of polarization directions in momentum space. As a conserved quantity, the topological charge would last and continuously evolves with varying parameters, only if it drops out of the continuum, or annihilate with an opposite charge. The evolution of topological charge leads to a series of novel phenomena, which broadens the horizon of fundamental physics, as well as will boost the applications in many areas, such as on-chip lasers, sensors, and photonic integrations.