Spinal arthrodesis represents one of the most frequent surgical procedures performed by spine surgeons with the ultimate objective to obtain an intervertebral fusion, i.e., bony bridge, between two or more adjacent vertebral segments. The absence of bony consolidation, i.e., pseudarthrosis or nonunion, may result into chronic pain, implant loosening and/or migration, instrumentation failure, and so on. It is therefore essential for spine surgeons to perfectly understand the optimal biological and mechanical environment that may promote spinal fusion. The aim of this chapter is to describe the mechanical and biological considerations that may impact bony healing during spinal fusion and instrumentation, helping the spine surgeons to optimize their fusion procedures. It is therefore divided into four main parts: the fundamentals/terminology, bony remodeling and healing, graft materials, and spinal fusion.