Mitochondrial diseases are caused by dysfunction of the mitochondrial oxidative phosphorylation system and can be the result of mutations both in mitochondrial DNA and in nuclear DNA. Mitochondrial diseases collectively describe a diverse group of heritable disorders, which may present at any age and have a wide spectrum of clinical manifestations. This leads to highly variable presentations, making the diagnosis of mitochondrial diseases challenging. Recent advances in genetic testing and novel reproductive options hold great promise for improving the clinical identification and treatment of mitochondrial diseases. In this work we discuss what is new in understanding and diagnosis of mitochondrial diseases.