Large volumes of wastewater are generated in the petroleum industry during oilfield extraction and at various refinement steps. Though the composition of the wastewater generated varies based on plant configuration and operational conditions, its characteristics are predominantly influenced by the organic and inorganic components of crude oil, most of which constitute a significant source of pollution. Discharging petroleum effluents without proper treatment is hazardous not only to the ecosystem but also to human health due to the toxic and carcinogenic nature of some of these pollutants. Exploiting microorganism communities, biological treatment options such as activated sludge methods enable economical and efficient wastewater treatment. This chapter provides an overview of the activated sludge treatment methods employed in oilfield and petroleum refinery wastewater treatment and discusses their principles, components, and workflows. Furthermore, practical examples of the application of each of these methods in managing oilfield and refinery wastewater are presented, and the efficiency and effectiveness of these solutions are compared.