The 21-cm signal is a powerful probe of the high-redshift universe, offering a unique window into the cosmic dawn and the dark ages. In this talk, I will introduce the fundamentals of 21-cm cosmology and highlight its advantages in studying the early universe. Through specific examples, I will illustrate how the 21-cm signal can be used to constrain the nature of dark matter, provide independent confirmation of an excess radio background, and make predictions for galaxy surveys. Finally, I will present ECHO21, a new Python package designed to efficiently generate a large suite of models spanning the dark ages and cosmic dawn.