1. Introduction Biped locomotion consists of both sagittal and lateral (frontal) plane motions. Although the stability of the locomotion must be ensured in both the planes, their natures are different. In the sagittal plane, the main purpose is to move from one place to another; thus, the stability is dynamic – losing static balance is essential in sagittal plane motion; it produces tumble for travel. In the lateral plane, on the other hand, maintaining an upright posture is crucial. Hence, lateral stability is static, and stabilizing a saddle point in the phase plane of the inverted pendulum motion is the main challenge. In general, the zero moment point (ZMP) criterion is utilized for biped motion control (Kagami et al., 2002; Mitobe et al., 2001; Nagasaka et al., 1999; Suleiman et al., 2009; Yamaguchi & Takanishi, 1997). Although this method is effective and useful, planned motion using this method is not suitable when the environmental conditions change from those considered during motion planning. The literature offers excellent reports on the modification of planned motion (Hirai et al., 1998; Huang et al., 2000; Kulvanit et al., 2005; Lee et al., 2005; Napoleon & Sampei, 2002; Prahlad et al., 2007; Wollherr & Buss, 2004), or online motion generation (Behnke, 2006; Czarnetzki et al., 2009; Héliot & Espiau, 2008; Kajita & Tani, 1996; Nishiwaki et al., 2002; Sugihara et al., 2002) that solve this problem. Usually, motion planning based on the ZMP criterion is applied to both the sagittal and lateral planes. The concept of this paper is that motion planning in the lateral plane can be skipped because of the difference in the nature of its stability. In the sagittal plane, motion planning is certainly crucial: one cannot proceed without actively generating both leg swing and torso behaviour, followed by the planned motion. The ZMP method was originally proposed to design such co-ordinated motions. However, in the lateral plane, balance is the primary purpose; generating active motion is a secondary problem. Nonetheless, in the ZMP method, the motion is first planned, and balance is maintained as a result of exact tracking of the planned motion. In our opinion, the process should be reversed for motion in the lateral plane, with balance control coming first and motion emerging as a result of balance control. From this viewpoint, trajectory generation for the lateral plane should be eliminated by setting balance as the control object. Satoshi Ito and Minoru Sasaki Gifu University Japan Motion Control of Biped Lateral Stepping Based on Zero Moment Point Feedback for Adaptation to Slopes 2