The complementary feeding period (6-24 months) is a window of opportunity for preventing stunting, wasting, overweight, and obesity and for improving long-term development and health. Because WHO published its guiding principles for complementary feeding in 2003, new knowledge and evidence have been generated in the area of child feeding. The aim of this paper is to highlight some of the emerging issues in complementary feeding and potential implications on the guidelines revision. Evidence on the effect of the quality and quantity of protein and fat intake on child growth during the complementary feeding period is summarized. The increased availability of sugar-containing beverages and unhealthy snack foods and its negative effect on young child's diet is described. Negative effects of nonresponsive feeding and force feeding are also discussed, although few scientific studies have addressed these issues. There are several emerging research areas that are likely to provide a better understanding of how complementary feeding influences growth, development, and health. These include the effect of the young child's diet on body composition, gastrointestinal microbiota, and environmental enteric dysfunction. However, at present, findings from these research areas are not likely to influence guidelines. Several emerging issues will be relevant to address when complementary feeding guidelines will be updated. With the increasing prevalence of obesity globally, it is important that guidelines on complementary feeding address both prevention of undernutrition and prevention of overweight, obesity, and noncommunicable diseases later in life.