Population is the total number of people (or animals, or plants) living in a particular place. When news reports say “the population of California is about 39 million,” they mean that's how many people live there. Scientists studying wolves might count the wolf population in Yellowstone National Park to see if their numbers are growing or shrinking.

Understanding populations helps us plan for the future. Cities need to know their population to figure out how many schools, hospitals, and fire stations they need. Countries track population to understand how their society is changing. When you hear that a town's population is booming, it means many new people are moving there. When a species' population drops dangerously low, conservationists work to protect it.

Population also involves understanding communities: who lives where, how many resources they need, and how groups change over time. The human population of Earth is about 8 billion people and growing, which raises important questions about how we'll share our planet's resources in the decades ahead.