In this 10-part series, which will run every other month through 2008 and 2009, "EP" will explore Attention Deficit Hyperactivity Disorder (ADHD). ADHD is technically defined as a neurodevelopmental, biological condition characterized by three hallmark symptoms: inattention, hyperactivity, impulsivity. Each of these symptoms has a special meaning to ADHD experts. According to a well-recognized authority (the Diagnostic and Statistical Manual of Mental Disorders, DSM-IV), there are three subtypes of ADHD: ADHD, predominately inattentive type (formerly known as ADD); ADHD, predominately hyperactive type; and ADHD, combined inattentive and hyperactive type. This article discusses the three subtypes of ADHD and describes four conditions that often coexist with ADHD, namely: (1) depression; (2) anxiety; (3) learning differences; and (4) sleep disorders.