This study aims to provide new knowledge on the principles of treadmill walking with body weight support (BWS) by qualifying and quantifying the effects on gait in 28 healthy individuals. Gait assessment included measuring trunk movements during different conditions. Walking overground and on a treadmill with and without harness, and on a treadmill with 30% dynamic and static body weight support, was assessed. Gait variables for all conditions were compared at 1.2m/s. On the treadmill, cadence increased, the trunk tilted more forwards, vertical acceleration increased and anteroposterior acceleration became more variable. Wearing a harness resulted in more restricted vertical acceleration. Walking with body weight support restricted acceleration in all directions. It also produced variable interstride trunk acceleration in the anteroposterior and vertical directions, but more consistent interstride trunk acceleration in the mediolateral direction. Static BWS gave larger differences than dynamic BWS.