Many medical conditions rely upon strict numerical definitions to provide a diagnosis; diabetes mellitus and hyperlipidemia are two examples. In the case of brow ptosis, diagnosis is determined predominantly by the judgment and experience of the examining physician. Brow ptosis exists when inferior malposition of the brow interferes with aesthetics or function. The brow level deemed low in one person may be perfectly acceptable or "normal" in another.

With the brow being a mobile structure and prone to the secondary effects of age, solar elastosis, muscle action, trauma, and gravity, some degree of brow descent will eventually occur in everyone. Ideal brow position is regarded differently in different genders, races, ages, and even generations. In some communities, the concept of changing the brow's position or shape is considered anathema; in many Western societies, however, it is considered routine.

The classic teaching describes the ideal female brow position as above the level of the bony supraorbital rim, with an upward arch such that the peak of the brow lies between the lateral limbus and the lateral canthus. In men, the eyebrows normally sit at or just above the superior orbital rim, with a flatter contour. Age, cultural influences, occupations, and environmental effects all influence not only brow position and shape but also perceptions of what is aesthetically pleasing. A weather-worn farmer, for example, may have an inferiorly-positioned brow that provides some protection from light, dust, and wind. On the other hand, a model may require a higher brow position in order to appear more youthful or attractive, regardless of gender. Subtle changes in brow shape are also indicators of emotional state: low lateral eyebrows denote sadness or concern, low medial brows indicate anger, flat or low brows may display fatigue, and excessively elevated brows appear surprised. Similarly, temporal hooding and upper eyelid dermatochalasis may indicate tiredness, but when combined with frontalis overactivation because of the heavy upper eyelids, the impression of fatigue is multiplied. Finding the precise balance to portray happiness and vitality can be challenging. If upper blepharoplasty and blepharoptosis repair take place without addressing brow ptosis, the brows will appear lower after surgery because the frontalis tone is diminished once the visual fields are improved, thus also exacerbating a fatigued appearance. When brow ptosis is present, it is rarely completely symmetrical, because of myriad factors, including differences between the right and left sides of the face (hemifacial microsomia or facial paralysis), differential exposure to the elements (particularly for those who drive with a lowered window), the preferred side a patient may sleep on, and many others all affect brow position.