Trans-trabecular meshwork surgeries attempt to lower intraocular pressure (IOP) by shunting across the trabecular meshwork (TM) that is believed to generate the bulk of aqueous humour outflow resistance. Despite elevated TM resistance in glaucoma, newer trans-TM surgeries appear to be less successful at lowering IOP compared to traditional filtration surgeries (Mosaed et al. Trans. Am. Ophthalmol. Soc., 2009). To understand the mechanism and possible limitations of trans-TM surgeries, we review the anatomy and basic science of outflow resistance generation within the TM and post-TM aqueous vessels. These studies provide estimates of IOP expected from trans-TM shunts and reveal obstacles that may limit the efficacy of trans-TM surgeries.