Drusen are small, extracellular deposits that accumulate below the retinal pigment epithelium on Bruch's membrane.⁷ “Hard” drusen, including basal laminar drusen and cuticular drusen, represent focal thickening of Bruch's membrane or the basal membrane underlying the retinal pigment epithelium (RPE). “Soft” drusen are usually larger (small RPE detachments) and carry a higher risk of degenerative changes.³⁷ Although drusen are located in the posterior pole, they are biomarkers of a more diffuse disease process that is complex and at least partly genetically determined. Drusen are usually a hallmark of a progressive macular degeneration process, and their formation parallels changes in Bruch's membrane and the RPE. The deposits seen in dominant drusen are distinguished from those seen in Stargardt's disease, Best disease, or other “flecked” retinopathies. These pathologies can be characterized on the basis of clinical and pathophysiological findings. These conditions will not be discussed in this chapter, nor will optic nerve drusen, also a distinct entity. Drusen are usually seen in the aging eye but can be seen as early as the first decade, especially when a hereditary pattern is documented. These changes are usually not associated with any systemic findings, although drusen can be documented in conditions such as mesangiocapillary glomerulonephritis type II.¹¹