Cell-free (CF) multiple-input multiple-output (MIMO) is a promising technique to enable the vision of ubiquitous wireless connectivity for next-generation network communications. Compared to traditional co-located massive MIMO, CF MIMO allows geographically distributed access points (APs) to serve all users on the same time-frequency resource with spatial multiplexing techniques, resulting in better performance in terms of both spectral efficiency and coverage enhancement. However, the performance gain is achieved at the expense of deploying more APs with high cost and power consumption. To address this issue, the recently proposed reconfigurable intelligent surface (RIS) technique stands out with its unique advantages of low cost, low energy consumption and programmability. In this paper, we provide an overview of RIS-assisted CF MIMO and its interaction with advanced optimization designs and novel applications. Particularly, recent studies on typical performance metrics such as energy efficiency (EE) and spectral efficiency (SE) are surveyed. Besides, the application of RIS-assisted CF MIMO techniques in various future communication systems is also envisioned. Additionally, we briefly discuss the technical challenges and open problems for this area to inspire research direction and fully exploit its potential in meeting the demands of future wireless communication systems.
