This chapter will briefly introduce the research background of Optical Neural Networks (ONNs), while also reviewing the developmental trajectory of ONNs. As a novel computing paradigm, ONNs possess inherent advantages such as low latency, low power consumption, and ultra-fast parallel signal processing compared to electronic neural networks. Although ONNs experienced slow development in the past due to limitations in fabrication technology, societal demands, and data resources, recent advancements in these areas have facilitated rapid progress in ONNs. This chapter will provide a detailed overview of the current research status of ONNs, analyze potential areas for improvement in different types of ONN studies, and present the research content, significance, and objectives to be explored in this thesis.