Among the several microstructural parameters that affect the properties of materials, grain boundaries (GBs) are prominent. Grain boundary character and distribution have been shown to influence interface-controlled properties such as creep, fatigue, fracture, segregation and intergranular corrosion. The concept of grain boundary engineering (GBE) was proposed in 1980s in which the presence (above a critical fraction) of low energy, damage tolerant coincident site lattice (CSL) grain boundaries (especially Σ3 boundaries) resulted in an improvement in the performance of the material. This chapter presents briefly the fundamental principles underlying grain boundary geometry, characterization, and measurement, with a specific focus on coincident site lattice boundaries. Subsequent sections elucidate the operative mechanisms during thermomechanical processing widely employed to realise GBE microstucture namely strain-annealing and strain-recrystallisation. In the context of GBE, the chapter highlights the formation and evolution of annealing twins, which are pivotal for achieving an ideal GBE microstructure due to their prevalence among CSL boundaries. The interactions between twin boundaries and their higher-order equivalents, leading to the grain boundary network, often referred to as twin-related domains are also discussed. Quantification of microsctructural parameters that define a GBE microstructure is essential and the chapter outlines the methodologies for quantifying such microstructures. The role of coherency of twin boundaries and the significance of the grain boundary plane in GBE is briefly discussed. The chapter reviews various modeling and simulation techniques employed to study GB network evolution during thermomechanical processing within the context of GBE following which emerging trends in characterization, along the challenges and future directions in grain boundary engineering research are presented.