Horses are transported more frequently than any other type of livestock. Most of our understanding about horse transport has been based on custom and conjecture, but some recent studies have greatly increased our knowledge base. Although some horses adapt to transport much better than others, transport is generally associated with lower reproductive rates, increased disease incidence, a temporary reduction in athletic performance, and the alteration of many other physiological parameters that are indicative of stress. Horses show marked dehydration after 24 h and extreme dehydration after 28 h of transport in hot and humid conditions where there is little nighttime cooling. Watering horses onboard trailers will alleviate dehydration, but fatigue can become extreme after 28 h of transport. Orientation either toward, away or diagonally from the direction of travel does not appear to significantly affect a horse's ability to maintain its balance. Allowing horses the ability to raise and lower their heads or hind quarters and to take at least one step in any direction appear to be the most important factors in their compensating for changes in inertial forces. Loose horses that are transported in groups at high densities, e.g. slaughter horses, do not hold each other up, but inhibit each other's attempts to compensate for changes in inertial forces. High density also increases injuries and inhibits the ability of horses to stand once they fall due to other horses standing over the downed horses. High density also prevents submissive horses from moving away from aggressive horses, resulting in repeated aggression. However, reducing density will greatly increase transportation costs and recent surveys have shown that many of the worst injuries seen on horses arriving at slaughter plants originated prior to their transport to slaughter. Horse trailer design has undergone rapid change with the rise in popularity of slant-load aluminum trailers. A major challenge for the future is determining which of the myriad of trailer designs, suspension systems and building materials available to horse owners and shippers are preferable.