**1. Introduction**

Multi-Robot Systems (MRSs) have emerged as a suitable alternative to single robots to improve current and enable new missions. These systems offer the following advantages over single robots:


However, multi-robot systems still face challenges related to robot autonomy and human factors. The deployment, operation, and collection of these systems in real-world scenarios need autonomy in the broad sense: robots with more capabilities and intelligence to operate longer in adverse conditions. In addition, the complexity of these systems poses some challenges to operators in terms of workload, situational awareness, and stress.

The recent literature on MRSs considers these challenges and proposes new strategies to face them. That is the case of artificial intelligence, which has given rise to new algorithms that allow managing the complexity and uncertainty of real scenarios, and immersive technologies (virtual and augmented reality), which are applied to facilitate the work of operators. These technologies are opening up a wide variety of missions, such as search and rescue, environmental monitoring, and many more.

In this "Special Issue on Multi-Robot Systems: Challenges, Trends, and Applications", we have collected a set of high-quality works that discuss the main challenges of MRSs, present the trends to address these issues, and report various relevant applications.

The remainder of this editorial is organized as follows: Section 2 discusses the challenges of MRSs, Section 3 addresses the proposals to solve them, and Section 4 describes the real-world applications presented in the different articles of the Special Issue.

**Citation:** Roldán-Gómez, J.J.; Barrientos, A. Special Issue on Multi-Robot Systems: Challenges, Trends, and Applications. *Appl. Sci.* **2021**, *11*, 11861. https://doi.org/ 10.3390/app112411861

Received: 20 October 2021 Accepted: 30 November 2021 Published: 14 December 2021

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