This chapter examines the dynamic nature of ocular measurements and the wide array of environmental and physiologic factors that influence them. Key parameters such as intraocular pressure (IOP), central corneal thickness (CCT), and retinal thickness demonstrate diurnal variation, with notable fluctuations based on time of day. Postural changes, physical activity, and Valsalva maneuvers can transiently affect IOP and optic nerve appearance, relevant for glaucoma management. Additional influences include dietary substances (e.g., caffeine, alcohol, salt), hydration status, and hormonal changes (e.g., pregnancy, menstrual cycle), all of which may modulate ocular physiology. External conditions such as altitude, temperature, and humidity also impact ocular blood flow and tear film stability. Systemic factors like hypertension, sleep apnea, and stress can further alter ocular parameters. Accurate interpretation of ophthalmic measurements requires awareness of these variables, reinforcing the importance of consistent timing, documentation, and repeat assessments in clinical practice.