Inclusion Criteria:
* Male and female adults 18-65 years of age.
* A diagnosis of childhood onset ADHD, meeting the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) \[American Psychiatric Association, 2013\] criteria, including at least 5 moderate inattentive or impulsive-hyperactive symptoms, and onset of several symptoms of inattentive or of impulsive/hyperactive traits by the age of 12.
* English-speaking.
Exclusion Criteria:
\- Current or past history of mental retardation, severe sensory impairment such as deafness or blindness, hypomania, mania, psychosis, suicidal or homicidal behavior.
Current moderate to severe symptoms of a mental health condition other than ADHD, assessed using a clinical evaluation and the Adult Self Report Scale that, in the judgment of the investigator, may jeopardize subject safety or interfere with their ability to participate in the study. Specifically, this will include current clinical diagnosis of moderate to severe major depression, or a score on the depressive problem subscale of the ASRS that falls in the clinically significant range.
* Substance use disorder within the past 6 months.
* Any significant medical condition, that, in the judgment of the investigator, may jeopardize subject safety.
* Pregnant females.
* Inability or unwillingness to participate in study procedures.
* Contraindication to tDCS: history of epilepsy, metallic implants in the head and neck, brain stimulators, vagus nerve stimulators, VP shunt, or pacemakers.
* Skin conditions that may make the application of, treatment with, and removal of the tDCS hardware painful as per the discretion of the clinician.
* Current use of a medication considered to be therapeutic for ADHD. If a subject is taking a medication that is considered by study investigators to potentially treat ADHD (eg. a stimulant, atomoxetine, buproprion, modafanil, serotonin-norepinephrine reuptake inhibitor, guanfacine or clonidine) they must stop use of this medication for at least 5 half lives of the drug under physician guidance prior to study participation. Subjects will not enter the study if it would require stopping a medication that is optimally and comfortably managing a clinical concern.