A very high proportion of both children and adults with ADHD have at least one other psychiatric disorder. These comorbid disorders can be dynamic, implying that they may change with age or may wax and wane over the lifetime of the individual. Subthreshold psychiatric disorders, that do not meet strict diagnostic criteria but nevertheless impair the quality of life of the individual significantly, can also occur with ADHD. It is suggested that ADHD is a foundational disorder underlying many other psychiatric conditions and increasing the vulnerability to them.