Hamburger-Hamilton (HH) stages are a system used by those who study chick embryo development to describe the apparent maturity of the developing chick from laying of the egg to hatching. An embryo is assigned an HH stage (numbered from 1 to 46) based on its physical shape and visible features (morphology). This staging system is not dependent on the chronological age nor the size of the embryo, which are susceptible to variations due to breed, temperature during incubation, season, and time between egg laying and incubation. The stages, are in a sense, arbitrary levels of maturity based on multiple physical features. For example, embryos that might have different ages or sizes might be assigned the same HH stage based on their external appearance due to the natural variation which occurs between individuals or due to differing conditions during incubation.

The following morphological landmarks, when present, are examples of what is considered when assigning HH stages to a chick embryo: the shape and size of the primitive streak; the shape and condition of the neural tube (developing brain and nervous system); the presence and number of somites in the paraxial mesoderm; the appearance and characteristics of the branchial arches as they form the jaw, larynx and pharynx; the appearance of the limbs and the formation of wing buds, joints, and digits; the formation of eyelids, feathers and beak.