This chapter focuses on the assessment of wastewater treatment plants (WWTPs) and the various techniques and methodologies used to evaluate their condition and performance. Several techniques are employed, including physical inspections, maintenance records review, process monitoring and control, performance evaluation and benchmarking, structural and mechanical integrity assessments, advanced data analysis and modeling, and environmental and health impact assessments. Physical inspections and maintenance records review are crucial for identifying signs of wear, damage, or systemic problems that may affect the plant’s operation. Process monitoring and control, including online monitoring and laboratory analysis, help ensure optimal operating conditions and assess treatment efficiency. Performance evaluation and benchmarking involve assessing treatment efficiency and capacity utilization, and comparing performance indicators against industry standards or similar plants. Structural and mechanical integrity assessments utilize non-destructive testing techniques and comprehensive evaluation programs to prioritize maintenance and rehabilitation efforts. Advanced data analysis and modeling, such as data analytics and hydraulic and process modeling, are employed to analyze historical operational data, predict future performance, and optimize process design and operation. Environmental and health impact assessments, including life cycle assessment and public health impact studies, evaluate the environmental impacts of WWTPs throughout their lifecycle and assess the effectiveness of pathogen and toxic compound removal to minimize health risks. Compliance audits and regulatory reviews ensure adherence to environmental regulations, permits, and standards. Overall, this chapter emphasizes the importance of conducting comprehensive assessments that consider environmental and operational aspects to enhance sustainability and flexibility in wastewater treatment.