In the last decades, moving-bed biofilm reactors (MBBRs) have found a niche in the treatment of municipal and industrial wastewaters due to advantages of compactness, simplicity, stability and increased reaction rates. Recently, the material surface properties of MBBR carriers and their modification have been investigated, as reviewed herein, in order to enhance the control of microbial attachment and biofilm development, and MBBR performance by faster reactor startups or increased specific activity per surface area. Biofilm formation is a complex process influenced by the solid substratum surface properties and microbial composition and characteristics. MBBR carriers can be made to facilitate biofilm formation by modifying their physico-chemical surface characteristics using blended polymers, mixed materials, coatings and incorporating different chemical functional groups. The chemical modification of the substratum polymeric surface for biofilm treatment reactors has comprised plasma oxidation treatment, radiolysis in a gas phase, liquid-phase oxidative treatment and polymer grafting. This review focusses on carrier material surface properties, the modification of such properties and carrier material choices relevant to biofilm development and functionality of MBBRs, in order to identify opportunities and challenges in future biofilm carrier development.