The co-occurrence of epilepsy in people with intellectual disabilities (ID) and other developmental disabilities (DD) has received attention because it has a significant negative impact on health, well-being, and quality of life. The current research investigating the frequency and form of epilepsy in children with ID and DD is reviewed, with particular attention is paid to children presenting with coexisting autism spectrum disorder (ASD) and attention deficit hyperactivity disorder (ADHD). Complications in assessment and treatment arising from comorbidities are considered. The most recent and epidemiological survey of children with epilepsy demonstrates that approximately one-fourth of patients with childhood epilepsy had ID. The prevalence of DD (including ADHD and ASD) in children with epilepsy ranges from 7.1% to 32%, which exceeds that of the general population. Multiple studies have suggested that ADHD in children with epilepsy is largely comorbid rather than cause and effect because it is already present before the onset of epilepsy in 80% of these children. However, the co-occurrence of epilepsy and ASDs is frequent in people with known organic brain disorders, in which several causal relationships have been speculated. Recent studies have shown that the severity of ID is an important factor in determining the incidence of epilepsy complications in those with ASD, and also the incidence of ASD complications in those with epilepsy. The co-occurrence of epilepsy, ID, and DD appears common. ID might play a key role in increasing the burden of epilepsy, as well as DD comorbidity. The comorbidity of ID and DD in patients with epilepsy might reduce the seizure threshold, resulting in an increase in the frequency of seizures as well as epileptiform electroencephalographic abnormalities, although evidence is limited and further investigations are warranted.