Glaucoma is a group of progressive optic neuropathies in which the axons in the optic nerve are injured, retinal ganglion cell numbers are reduced and vision is gradually and permanently lost. The only approved and effective way to treat glaucoma is to reduce the intraocular pressure (IOP). This is usually accomplished by surgical and/or pharmacological means. Drugs designed to reduce IOP target one or more of the parameters that maintain it. These parameters (collectively known as aqueous humor dynamics) are the production rate of aqueous humor, the pressure in the episcleral veins and the drainage of aqueous humor through the trabecular or uveoscleral outflow pathways. Intraocular pressure lowering drugs can be classified as inflow or outflow depending on whether they reduce aqueous humor inflow into the anterior chamber or improve aqueous humor outflow from the anterior chamber. Inflow drugs, like β adrenergic antagonists and carbonic anhydrase inhibitors, reduce the rate of aqueous humor production. Outflow drugs, like prostaglandin analogs, cholinergic agonists and sympathomimetics, increase the rate of drainage through the uveoscleral outflow pathway and/or increase the facility of outflow through the trabecular meshwork. Some drugs have mixed inflow/outflow effects. This review summarizes the pharmacological treatments for glaucoma in use today and some new drugs showing potential for use in the future.