Inclusion Criteria for CADASIL Participants:

1. Must be at least 18 years old
2. Positive NOTCH3 genetic testing; OR a positive skin biopsy; OR a willingness to have a NOTCH3 genetic test completed prior to enrolling AND are at-risk for, or diagnosed clinically with, CADASIL
3. Willing to commit to three in-person visits (a baseline visit, an 18-month follow-up, and a 36-month follow-up) and to remote visits as needed by phone, email, mail or internet
4. Willing to provide documentation of all current medications to study team

a. All medications will be allowed throughout the course of study. Documentation of medications will be used for analyses to assess potential impact of medications on study outcomes.
5. Willing and able to undergo an MRI scan and blood draw at each in-person visit
6. Must have a designated "study companion"

a. A "study companion" is someone who knows the participant well (has greater than or equal to 3 hours/month of contact with the CADASIL participant) and can provide additional information to the study team (either remotely or in-person).
7. A functional capacity less than 4 on the Modified Rankin Scale

Inclusion Criteria for Healthy Controls (HC):

1. Must meet same criteria as CADASIL participants, EXCEPT have negative NOTCH3 genetic testing

Exclusion Criteria:

1. History of severe learning disability, intellectual disability, or other neurological disease or event not attributable to CADASIL
2. History of serious alcohol or drug abuse within the past year
3. Unwilling to undergo NOTCH3 genetic testing if there is no test on file