Mitochondrial diseases are a group of diseases caused by dysfunctional mitochondria, organelles that generate energy for the cell. Mitochondrial diseases are often caused by mutations, acquired, or inherited in the mitochondrial DNA or nuclear genes that code for respiratory chain complexes in the mitochondrion. Mitochondrial diseases involve multiple organs and show heterogeneous and unpredictable progression. The most common clinical presentation of mitochondrial diseases is encephalomyopathy, and epileptic seizures can frequently occur as a presenting sign of mitochondrial encephalopathy. While whether mitochondrial dysfunction or epilepsy is the cause or consequence is still debatable, they may be interrelated to create a vicious cycle. Epileptic phenotypes vary in different mitochondrial diseases. At present, there are no curative treatments for mitochondrial diseases, and the efficacy of many anticonvulsants, vitamins, nutritional supplements, and the ketogenic diet remain to be proven. Understanding the pathophysiology of mitochondrial diseases may further facilitate effective diagnostic and therapeutic approaches to these diseases.