As the name implies, an epileptogenic focus is a seizure-causing lesion, i.e., a lesion capable of causing abnormal neuronal discharges in the brain that result in seizures. From a neurophysiological perspective, it is the site or sites where the onset of epileptic discharges or the most pronounced epileptic discharges are seen on the EEG. Because of the multiple possible causes of epilepsy, there is not necessarily only one number of epileptogenic foci in the brain; often, multiple epileptogenic foci are more likely to be present in patients with primary epilepsy; in patients with secondary epilepsy, a single epileptogenic foci is more likely to be present. Some patients with epilepsy can have diffuse discharges without an obvious epileptogenic focus in the brain, mostly in primary epilepsy with an obvious genetic cause. This is because the etiology of this type of epilepsy is due to abnormal function of some ion channels in the cell membranes of nerve cells and nerve cell dysfunction. Therefore, the neuronal discharges are widespread without forming specific epileptogenic foci. In addition, some systemic or metabolic diseases causing epilepsy may also have diffuse neuronal discharges. The precise localization of the epileptogenic focus determines whether the patient can have surgery, the surgical approach and the surgical outcome, so it is very important for the patient to be operated on! What do doctors rely on to accurately locate the epileptogenic focus? (1) Symptoms: such as what the seizure looks like, when the seizure occurs (day, night; sleep, awake; when sleeping, when waking up, etc.), whether there is only one fixed style of seizure or two or more styles of seizures, etc. (2) Signs: paralysis, visual field changes, speech impairment, etc. (3) Scalp EEG during seizures: commonly known as seizure capture. At this time, long-range video EEG should be performed for two purposes: first, to record the patient’s performance during seizures in detail; second, to determine the site of origin of epileptiform discharges during seizures. These two items are crucial in the localization of the epileptogenic focus. Therefore, seizure capture is mandatory for surgical patients, meaning that seizure capture cannot be omitted for patients proposed for surgery. The process of seizure capture is: on board (general EEG performed) → various approaches to induce seizures: discontinuation of antiepileptic drugs or drug reduction; flash stimulation; sleep deprivation or even drug-induced seizures; etc. → capture of ≥3 seizures of each style → off board. Several issues need to be clarified when capturing seizures: (1) long EEG monitoring may be required because some patients have fewer seizures or ineffective induction; (2) seizures are usually very frequent, but fewer seizures after bringing the EEG; (3) the more types of seizure styles, the more seizures need to be captured and the longer the time; (4) the number of seizures is sufficient, but still insufficient to determine the location of the epileptogenic focus. (4) Structural brain imaging: including head CT, magnetic resonance imaging (MRI), cerebral angiography (DSA), etc. (5) Functional brain imaging: including functional MRI, PET-CT, magnetoencephalography (MEG), etc. (6) Intracranial buried electrode: It is to capture seizures after the electrode is placed on the surface of brain tissue or inserted deep into the brain tissue by opening the skull. Doctors localize the epileptogenic focus on the basis of comprehensive thinking and analysis of several of the above elements or test results. For most patients, the first 4 items are basically able to determine the location of the epileptogenic focus. If the exact location of the epileptogenic focus cannot be determined, the fifth test will be performed; if the fifth test still cannot determine the location of the epileptogenic focus, the sixth test will be performed. In some patients, the location of the epileptogenic focus cannot be determined due to the fact that the abnormal discharge is diffuse and there is no centralized area in the brain. In summary, it can be seen that: 1) each step of epileptogenic focus localization has uncertainty. In other words, each test may be inconclusive (it is not entirely correct to interpret it as a waste of money, but there is some truth to it); ② craniotomy with buried electrodes to capture seizures is currently the most accurate and last method to localize the epileptogenic focus. However, it has three obvious disadvantages: first, the location of the epileptogenic focus cannot be determined in a very small number of patients with buried electrodes to capture seizures (0.5-1%); second, it is an invasive test and a smaller craniotomy with certain risks; and third, it is expensive. In actual clinical practice, localization of the epileptogenic focus is often the most difficult. For patients with difficult localization of the epileptogenic focus, it often requires a comprehensive discussion among physicians in epilepsy medicine, epilepsy surgery, and EEG together to determine it.