Inclusion Criteria:  individuals must be age 18-75  a military veteran or active military service member who has deployed or is scheduled to deploy in support of post-9/11 operations, such as:  Operation Iraqi Freedom Operations Enduring Freedom Operation New Dawn  there are no additional inclusion criteria for Aims 1 and 2  however, for the Supplemental Aim, to be eligible to participate all interested veterans must have a diagnosis of PTSD (i.e., current or lifetime)  Exclusion Criteria:  individuals will be excluded for a history of neurological illness or cognitive disorders other than TBI (e.g., Huntington's, Parkinson's, dementia, or Multiple Sclerosis), and non-TBI related seizures additionally, individuals are excluded for a current diagnosis of psychotic disorders, schizophrenia, bipolar disorders, current suicidal or homicidal ideation/intention/planning  there are no additional exclusionary criteria for Aims 1 and 2  however, for the Supplemental Aim, any participants with an absolute or relative contraindication to exercise testing as recommended by the American Thoracic Society and the American College of Chest Physicians (e.g., acute myocardial infarction, unstable angina, respiratory failure, uncontrolled asthma, orthopedic impairments that compromise exercise performance, uncontrolled heart failure and acute hypertension) will be excluded from the cardiorespiratory exercise test