Medication Adherence in Children Who Had a Liver Transplant

Inclusion Criteria: * The patient is between \> 1 year of age and less than 18 years of age at enrollment * Guardian's consent, child assent (in accordance with each institution's IRB policies). * The patient is prescribed tacrolimus (either brand or generic formulation). * The patient has been seen in the enrolling center's clinic at least once in the last two years. Exclusion Criteria: * The patient received a liver transplant less than 1 year prior to enrollment. * The patient has had more than one solid organ transplant (including marrow replacement). * The patient has had biopsy-proven rejection within the past six months. * The patient has been diagnosed with Hepatitis C. * The guardian or child (in a developmentally-appropriate manner) do not understand the study procedures. This will be verified by asking both guardian and child (if 6 years old or older) to repeat the study procedures. * The patient is only seen for consultation - most or all of the child's routine care is provided at another center (or in a community clinic). * Either the patient or the guardian is actively psychotic or severely disoriented due to any cause, including hepatic encephalopathy (temporary exclusion) or severely mentally retarded as defined in the Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV). * The patient is not medically stable or is hospitalized. * The treating physician has instructed the participant not to obtain tacrolimus levels for at least one year.