This chapter provides an overview on the design and operation of biological nitrogen removal facilities. It reviews specific options and compares them in order to assist the user in selecting the most appropriate option for a particular situation. The chapter reviews process and facility design and discusses process operation. It discusses the extent of use of biological nitrogen removal and describes the performance capabilities of full-scale systems. The chapter focuses on single-sludge systems for combined carbon oxidation, nitrification, and denitrification. It includes separate stage denitrification systems on full-scale experiences. Two different process options are typically used for separate stage denitrification: suspended growth and attached growth. The chapter considers the procedure consisting of the following major elements: sizing of first aerobic zone, sizing of anoxic zones and mixed liquor recycle pumping, sizing of second aerobic zone, if needed, clarifier sizing and overall process mass balance check.