Abstract: Wire-based Directed Energy Deposition（W-DED） is an additive manufacturing technology that uses wire as the feedstock, characterized by its high efficiency, precision, and utilization rate. However, the process of melting and depositing the wire material may lead to the formation of deposition defects due to excessive heat input.Consequently, it is necessary to employ online monitoring techniques to observe the molten pool and deposited layers during the deposition process, thereby optimizing the control of the deposition process parameters. Multimodal signal fusion methods can enhance the accuracy and stability of monitoring tasks by utilizing physical information from different modalities. This paper first summarizes the relationship between the online monitoring targets of W-DED technology and the quality of the formed parts. It then categorizes the existing online monitoring techniques and compiles their related process control methods for deposition parameters. Lastly, the paper reviews the current research progress and challenges in W-DED concerning online monitoring and process control, and proposes future research directions.