Diagnosis in Medicine may be defined as "the recognition of a disease or condition by its outward signs and symptoms and the analysis of the underlying physiological/biochemical cause(s) of a disease or condition". Medical dignosis is the process of identifying a medical condition or disease by its signs, symptoms, and from the results of various diagnostic procedures. The conclusion reached through this process is called a diagnosis. The term "diagnostic criteria" designates the combination of signs, symptoms, and test results that allows the doctor to ascertain the diagnosis of the respective disease.

The physician typically tries to get information from patient about his/her present condition, history of the patient's illness and then physically examine him for signs of disease. The physician will formulate a hypothesis of likely diagnoses and in many cases will obtain further testing to confirm or clarify the diagnosis before providing treatment. Medical tests commonly performed are measuring blood pressure, checking the pulse rate, listening to the heart with a stethoscope, urine tests, fecal tests, saliva tests, blood tests, medical imaging, electrocardiogram, and occasionally biopsy. It is important not only to diagnose the disease but the cause of the disease. Advances in medicine helps us to find out the root cause as authentic cures can be obtained by correcting the causal abnormalities. If causalities are assumed to be unknown, then palliative treatments to reduce symptoms are the best treatments possible.

A physician's job is to know the human body and its functions in terms of normality (homeostasis). The four cornerstones of diagnostic medicine, each essential for understanding homeostasis, are: anatomy (the structure of the human body), physiology (how the body works), pathology (what can go wrong with the anatomy and physiology) and psychology (thought and behavior).

Once the doctor knows what is normal and can measure the patient's current condition against those norms, she or he can then determine the patient's particular departure from homeostasis and the degree of departure. This is called the diagnosis. Once the physician has completed the diagnosis, he explains the prognosis to the patient and proposes a treatment plan which includes therapy and follow–up (further consultations and tests to monitor the condition and the progress of the treatment, if needed), usually according to the guideline provided by the medical field on the treatment of the particular illness. Treatment itself may indicate a need for review of the diagnosis if there is a failure to respond to treatments that would normally work.