The main purpose of this study is to evaluate the safety and tolerability of LY3372993 in healthy participants and participants with AD. The study will also investigate how much LY3372993 gets into the bloodstream and test the effects of LY3372993 in participants with AD.

The study has two parts:

• Part A - Healthy participants will receive LY3372993 or placebo. Part A will last up to 17 weeks;
• Part B - Participants with AD will receive LY3372993 or placebo. Part B will last about 317 days.

In April, 2019, Eli Lilly and Company made a business decision to terminate Part B. No participants were enrolled.

Inclusion Criteria :

Part A:

• Overtly healthy males or females as determined by medical history and physical examination
• Are between 18 to 45 years old, inclusive
• Male participants agree to use a reliable method of birth control during the study and 3 months following the last dose of the investigational product
• Female participants not of child-bearing potential
• Have a body mass index of 18.0 to 32 kilograms per square meter (kg/m²) inclusive

Part B:

• Present with mild cognitive impairment (MCI) due to Alzheimer's disease (AD) or mild-to-moderate AD
• Positive florbetapir scan
• Men or nonfertile women, at least 55 years of age. Nonfertile is defined as hysterectomy and/or bilateral oophorectomy, or amenorrhea for at least 1 year
• Have up to 2 study partners who can provide health information related to the study about the participant. Study partner(s) will provide a separate written informed consent to participate

Exclusion Criteria:

• Have significant abnormalities in brain magnetic resonance imaging (MRI); or have contraindications for MRI
• Have significant allergic reactions to LY3372993, or related compounds, or have significant allergies to humanized monoclonal antibodies, diphenhydramine, epinephrine, or methylprednisolone
• Have clinically significant neurological or psychological illness, or other illnesses that could affect the study results

Part A:

• Have family history of early onset Alzheimer's Disease (AD)
• Have impaired cognitive function

Part B:

• History of intracranial hemorrhage, cerebrovascular aneurysm or arteriovenous malformation, or carotid artery occlusion, or stroke or epilepsy
• Previously dosed in any other study investigating active immunization against amyloid beta (Aβ)
• Previously dosed in any other study investigating passive immunization against Aβ within the last 6 months
• Have current serious or unstable illnesses