Ocular barriers effectively protect the eye from pharmaceuticals. The development of strategies to overcome these barriers in order to improve ocular drug delivery remains a major challenge for pharmaceutical sciences. The aim of the present work is to describe the current models of the ocular barriers (cornea, conjunctiva, sclera, blood-aqueous barrier, blood-retinal barrier). In vitro cell culture models have been used to study passive drug permeation and active drug transport across different ocular barriers as well as for toxicological screening of compounds as alternative to in vivo toxicity tests. Primary cell cultures and immortalized cell lines as in vitro models of ocular barriers are reviewed herein. Primary cell cultures (e.g. cornea, conjunctiva, retina) of different origin (e.g. human, rabbit, rat, bovine, porcine) are described. lmmortalized cell lines derived from corneal epithelial, conjunctival epithelial and retinal pigment epithelial cells have some advantages (the tissue isolation and cell purification is not required, growing after a large number of passages, revive after storage in liquid nitrogen) as well as disadvantages (potential abnormal characteristics, different gene expression profile) over primary cells. However, much work still needs to be done in order to develop cell-based permeability models of the ocular barriers for large scale screening of pharmaceutical compounds and formulations. Ex vivo tissue models have been used to study drug permeation across freshly excised biological ocular barriers such as cornea, conjunctiva, sclera, retinal pigment epithelium, neural retina. The results of drug permeability experiments with excised animal tissue are often less reproducible (species related applicability problems) in comparison to cell-based permeability experiments. Although the tissue based models represent low-throughput assay, this model provides a better prediction of in vivo ocular drug bioavailability in comparison to cell-based models. ln vivo animal experiments have been used for pharmacokinetic studies and ocular tolerance evaluations. The experiments are usually conducted on rabbits as commonly used animal model. In most cases rabbits have to be sacrificed at each time-point to collect samples in ocular pharmacokinetic studies. Therefore, animal experiments are often criticized for ethical and economical reasons. In this paper the different modifications of the classical pharmacokinetic scudies are presented. The ocular pharmacokinetics can be studied by analyzing the drug concentrations into tear fluid, aqueous humor and different ocular tissues. Furthermore, the pharmacological or side effect can be determined as indirect measure of the ocular drug bioavailability. The commonly used Draize test for the ocular tolerance evaluation is briefly described.