1) Nasal drug delivery is a method of administering drugs through the nose for local or systemic effects, avoiding first-pass metabolism. It provides rapid drug absorption and improved bioavailability for some molecules. 2) Liquid formulations like solutions, suspensions, and sprays are most commonly used for nasal delivery. Powders can also be used with insufflators or dry powder inhalers. 3) Evaluation of nasal formulations involves in vitro diffusion studies and in vivo models using rats, rabbits, dogs, sheep, or monkeys to assess absorption and bioavailability. Ex vivo nasal perfusion models are also used.