The terms “epileptic encephalopathy” (EE) and “developmental and epileptic encephalopathy” (DEE) are used to describe complex neurological conditions characterized by seizures, severe EEG abnormalities, and developmental impairments. EE, as initially defined, describes epilepsy-related disruptions in cerebral function, while DEE incorporates developmental impairments driven mainly by genetic aetiologies, with epileptic activity exacerbating these deficiencies. The management of these conditions necessitates a multidisciplinary approach, with long-term EEG-polygraphic monitoring playing a pivotal role in diagnosis and therapeutic planning. West syndrome, a prototypical EE, exemplifies the interplay between clinical and electroencephalographic features, including hypsarrhythmia and epileptic spasms. Variations in EEG patterns often correlate with specific aetiologies such as cortical dysplasia, lissencephaly, or hemimegalencephaly. Advances in genetics have illuminated the underpinnings of many DEEs, linking mutations in genes like KCNQ2, CDKL5, and PCDH19 to distinct electroclinical syndromes. Moreover, conditions such as Lennox-Gastaut syndrome, encephalopathy related to status epilepticus during slow sleep (ESES), and myoclonic status in nonprogressive encephalopathies reveal the diversity of sceneries requiring tailored diagnostic and management strategies, steered by EEG interpretation. EEG findings not only aid in identifying specific syndromes but also serve as prognostic markers and therapeutic targets. This review highlights the indispensability of systematic EEG-polygraphic assessments in advancing our understanding and management of EE and DEE, emphasizing their central role in modern epileptology.