**Preface to "Symmetry in Structural Health Monitoring"**

Structural health monitoring refers to the strategy and process of damage diagnosis and the characterization of engineering structures. With the development of urbanization, various types of infrastructure and mechanical equipment provide people with convenient life services. This also shows the importance of structural health monitoring where symmetry is widely used. By analyzing the symmetry of the structure and using sensors to collect data, it is possible to study the performance of the structure itself. This is a hot topic in the current research. The collected data contain the structural random vibration and environmental noise; good equipment can improve the efficiency of data collection, while effective denoising methods with the knowledge of structural dynamics can extract the characteristic parameters of the structure from the data. In this Special Issue on symmetry, we mainly discuss the application of symmetry in various aspects of structural health monitoring. For example, by considering the health monitoring of a known structure by obtaining the structure's static or dynamic response, using different signal processing methods, including some advanced filtering methods, to remove the influence of environmental noise and extract structural feature parameters to determine the structure's safety. These damage diagnosis methods can be effectively applied to various types of infrastructure and mechanical equipment. For this reason, the vibration control of various structures and the knowledge of random structure dynamics should be considered, which will promote the rapid development of the structural health monitoring. The signal extraction and evaluation methods are also worthy of study. The improvement in signal acquisition instruments and acquisition methods improves the accuracy of the data. A good evaluation method will help to correctly understand the performance with different types of infrastructure and mechanical equipment.

> **Yang Yang, Ying Lei, Xiaolin Meng, and Jun Li** *Editors*
