Many organic chemicals including pharmaceuticals, personal care products, pesticides, and industrial chemicals are detectable at trace levels (µg/L range and lower) in the environment and are therefore regarded as micropollutants (MPs). MPs pose a risk to natural aquatic ecosystems and human health through their persistent, bioaccumulative, and toxic properties, and are often released into the environment through wastewater effluent. Wastewater treatment plants (WWTPs) can successfully remove bulk contaminants from water but fail to provide reliable and complete removal of MPs. This is due to the nature of biological processes in WWTPs where the removal of organic compounds relies on complex metabolic functions governed by dynamic wastewater microbial communities. Despite years of research, we still lack a fundamental understanding of the types, frequency, and variability of microbial community functions within biological wastewater treatment processes. A new perspective on wastewater microbial community functioning is now possible through the use of high-resolution mass spectrometry (HRMS), where thousands of metabolites can be simultaneously measured in wastewater samples. This innovation enables novel metabolomics analyses, where a broad range of small molecules (i.e., metabolites or MPs) can be measured in biological wastewater treatment systems. The overarching goal of this dissertation is to increase our understanding of wastewater microbial community functioning through complementary studies of micropollutant biotransformations and nontarget metabolomics. This goal was addressed in three research projects. Results from the first project identified functional groups that determine rates of micropollutant biotransformations performed by wastewater microbial communities. This was done with laboratory-scale experiments in which MP biotransformation rates and pathways were measured in microcosms seeded with four independent wastewater microbial communities. Results from the second project identified specific biotransformations that exhibit varying activity levels at daily timescales in a full-scale WWTP. This was done by coupling a two-week field sampling campaign with HRMS acquisitions and novel data mining techniques. Results from the third project describe the metabolic functioning of a wastewater microbial community in a full-scale WWTP and identify the frequency with which specific biotransformations are performed. Overall, results of this dissertation provide insights into wastewater microbial community functioning and establish a path forward for improved biological treatment of MPs in WWTPs.