Radiology is a medical specialty with the core function of obtaining, and interpreting, images by means of electromagnetic or acoustic energy transmitted through the body. Another, closely related specialty, nuclear medicine, obtains and interprets images produced by radioactive substances introduced into the body. A physician who has taken a residency in radiology is called a radiologist.
There are various lower-level programs that train technicians in performing X-rays, ultrasound, and other diagnostic imaging procedures. These are done only on the order of a physician, who will interpret the results.
It should be noted that various physician specialists are quite competent in diagnosing from imaging specifically for their specialty. Much orthopedic surgery, for example, is done with an X-ray or other image of the operative site kept in the constant view of the surgeon, who does not need a radiologist at his side to explain the image. The orthopedist will, for example, use the combination of his direct vision and the information from the X-ray to decide how to insert mechanical reinforcements to repair a fracture. Emergency physicians and cardiologists routinely do the first reading of an X-ray used in the evaluation of chest pain.
In the United States, the organization that accredits training programs and certifies radiologists is the American College of Radiology (ACR). It is possible to obtain joint certification, on completing the required education and examinations, in, for example, radiology and nuclear medicine. The requirements, in this case, are specified jointly by the American Board of Nuclear Medicine and the ACR.
Equivalent certifying bodies exist in developed countries, and some physicians choose to be certified in more than one country. This usually requires only one training program that both recognize, but two examinations. One radiologist commented on the difference between the ACR and the Canadian Association of Radiologists; the actual radiology examination in Canada is conducted by the Royal College of Physicians and Surgeons of Canada. 
Some countries have one-way or mutual reciprocity with other national certifications. For example, Australia and New Zealand certify with the FRACR (Fellowship of the Royal Australasian College of Radiologists), but accept radiology certificates from Canada, Eire, South Africa, the United Kingdom, and the United States. 
There are a number of specialties of radiology. The core competence is called diagnostic radiology, and requires the candidate to spend two years in a residency program in a broad clinical field, such as internal medicine or pediatrics. At that point, the resident can transfer to a radiology program, usually diagnostic radiology, although there is at least one experiment in an accelerated plan to subspecialize in vascular and interventional radiology. 
Subspecialization requires additional fellowship training beyond the basic radiology residency. Subspecialties recognized by the ACR include:
- hospice and palliative medicine
- nuclear radiology
- pediatric radiology
- radiation oncology
- vascular & interventional radiology
It is also possible to plan for multispecialty, as opposed to subspecialty, certification. Nuclear radiology can come from a combination of nuclear medicine and general radiology residencies, rather than fellowship.
Some of the subspecialties are interdisciplinary; radiology is one of several specialty tracks that can prepare one for the subspecialty fellowship. Hospice and palliative medicine is a good example. It is possible to come to some subspecialties of radiology through alternate preparation, such as a cardiology fellowship followed by a vascular and interventional radiology fellowship.
The definition of subspecialty has changed over the years, and some physicians are certified in subspecialties for which certificates are no longer offered. Their certification remains valid, although if they have a recertification requirement, it may need to be in the closest current subspecialty. 
Neuroradiology involves diagnosis of the skull, sinuses, mastoids, spinal, and head and neck structures, using techniques including plain X-ray, angiography, interventional procedures for the nervous system, myelography, and all other imaging techniques, including those not using ionizing radiation, including magnetic resonance imaging and ultrasonography.
Essentially, this is the joint specialty of radiology and nuclear medicine. It specifically includes skill with dynamic and static nuclear imaging of pathophysiologic processes, quality control of instruments for such imaging (e.g., bone densitometry, gamma camera (also called Anger camera, positron emission tomography (PET), radionuclide angiography (RNA), single photon emission computed tomography (SPECT)) radiopharmaceuticals, and quality control of nuclear imaging instruments.
Much like neuroradiology deals with the use of all modalities of diagnosis and intervention of the nervous system, pediatric radiology deals with the special uses of all modalities in children. Where pediatric radiology and neuroradiologies are considered secondary specialties, pediatric neuroradiology is a tertiary specialty.
This is the branch of medicine concerned with the treatment of cancer and certain other diseases with ionizing radiation. That radiation may be delivered by an external generator or radioisotope source, a radioactive implant (i.e., brachytherapy), or radioisotopes ingested by, or injected into, the patient.
Vascular & interventional radiology
Again, this is a secondary specialty with possibilities for additional levels of certification. It deals with the diagnostic and interventional modalities using radiation in diagnosis of disorders of the arteries, veins, and lymphatics. In principle, it includes all vascular and nonvascular imaging-directed interventional procedures. All modalities and techniques used in diagnostic and interventional procedures are also part of this subspecialty.
In practice, there may be additional tertiary or quaternary training. For example, cardiology is secondary specialty of internal medicine. Invasive cardiology, which involves imaging-directed diagnostic procedures of the circulatory system, such as cardiac catheterization, involves an additional fellowship, but interventional cardiology, which conducts procedures such as percutaneous transluminal angioplasty, or atherectomy using mechanical or laser instruments, requires yet additional training.
Teleradiology is not a true specialty, but it does involve additional skills, to direct persons at a remote location to position a patient in an imaging or interventional device at that place, then transmit the images to the (tele)radiologist for analysis. Of course, if the examination is real-time imaging, such as fluoroscopy, the radiologist will need to direct the process in real time, and if it is interventional, the radiologist will need to manipulate the controls over a computer network.
Beyond the regular medical licensing requirements, physicians who use ionizing radiation, from external sources or radioactive substances administered to the patient, will usually need an additional license from appropriate radiation safety organizations. In the United States, this is under the Department of Energy at the national level; there may be additional local requirements.
- American College of Radiology, Welcome to the ABR Website
- Khalili, Korosh (February 2000), "Canadian versus US radiology certification examinations: the 1999 experience", Canadian Association of Radiologists Journal
- International Medical Recruitment, Radiology
- American College of Radiology, DIRECT Pathway ( Diagnostic and Interventional Radiology Enhanced Clinical Training and Certification)
- American College of Radiology, Discontinued Specialties