The present chapter discusses bio-trickling filters (BTFs) as an effective and low-maintenance solution for treating various pollutants in wastewater. We overview the BTF structure, operation system, biofilm structure, and its development on carriers or media in the BTFs. The chapter explores the role of microorganisms that developed in the biofilms and their positive and negative impacts. Additionally, it summarises different techniques of adherence of microorganisms to develop biofilms and their impact on BTFs. It explains the processes of removing pollutants from wastewater using BTFs, including heavy metals, organic matter, odour, and greenhouse gases (GHGs). Factors affecting the BTFs will be described, such as changes in contact time, pH value, temperature, nutrients, water content, trickling liquid velocity, and their correlation to the uptake efficiency of biofilms.