The animal kingdom is the entire group of living things that scientists classify as animals. This includes everything from tiny insects and worms to fish, birds, reptiles, and mammals, including humans. If it moves around at some point in its life, eats food (rather than making it from sunlight like plants do), and has specialized cells that work together, it belongs to the animal kingdom.

Scientists organize all living things into groups called kingdoms. The animal kingdom is one of these kingdoms, sitting alongside the plant kingdom, the fungi kingdom, and several kingdoms of microscopic organisms. Within the animal kingdom itself, creatures are sorted into smaller groups based on what they have in common: vertebrates have backbones, invertebrates don't, mammals nurse their young with milk, birds have feathers, and so on.

When biologists talk about the animal kingdom, they're thinking about the incredible diversity of animal life on Earth: over a million known species, from jellyfish to elephants, all sharing certain fundamental traits that make them animals rather than plants or fungi. Understanding the animal kingdom helps scientists see connections between different creatures and trace how life has evolved over millions of years.