Memory is the tie that binds together thoughts, impressions, and experiences. Memory function is dependent on several mental or cognitive abilities using several brain systems. Many disease processes can lead to compromise of these systems. When a patient presents to the neurologist with memory loss, the patient or the family is frequently concerned about a neurodegenerative process or dementia. Dementia is often defined as impairment of memory and at least one other cognitive domain that leads to a decline in ability to perform activities of daily living.  A more simple definition of dementia requires a decline in more than one cognitive domain (e.g., memory, language, visuospatial, or frontal executive function) that interferes with one's ability to function independently.
When evaluating a patient with a concern of memory loss, the following questions should be considered:
Does the patient truly have memory loss or is there another cognitive problem causing the memory disorder?
What is the localization within the brain of the memory problem?
What etiologies are responsible for the memory disorder?
What is the temporal profile for the memory loss: acute (seconds/minutes/hours), subacute (days/weeks), or chronic (many months to years)?
Memory loss versus another cognitive problem
The first consideration is to determine whether the patient truly has memory loss or another cognitive problem.
Memory dysfunction can result from hippocampal lesions (short-term memory loss, e.g., Alzheimer dementia) as well as from lesions of brain structures involved in long-term storage (e.g., semantic dementia). In many cases, memory is encoded properly in the hippocampus, but patients have trouble retrieving the stored memory. This retrieval deficit is typically due to problems with frontal lobe function, often caused by white matter disease.
Some degree of memory loss occurs normally with aging. Normal aging leads to decreased ability for retrieval from a decline in frontal lobe function, but does not impact the activities of daily living.  Memory loss becomes particularly concerning when it affects function or activities of daily living.
It is important to differentiate transient, fluctuating disturbances in consciousness due to a delirium from an underlying memory disorder. The history, exam, and neuropsychological testing can all be helpful in distinguishing a primary memory disorder from a delirium or impairment in retrieval.
A pseudodementia related to depression may result in a clinical presentation suggestive of a memory disorder. Consideration should be made regarding the patient's affective state, because depressed patients may experience diminished concentration, sleep disruption, and mild impairments on delayed recall, which may manifest as a memory disorder. 
Patients with memory problems or complaints who do not fit a diagnosis of dementia because they are not functionally impaired are often referred to as having mild cognitive impairment (MCI). The deficits in patients with MCI, by definition, are not severe enough to interfere with their normal activities of daily living. The most common form of MCI is amnestic MCI, in which memory is the primary problem. About 50% of patients diagnosed with amnestic MCI will progress to dementia within 5 years. The rate of progression to dementia is about 12% per year.  However, MCI can occur in cognitive domains other than memory, including language, visuospatial, and frontal executive (e.g., problems organizing, planning, multi-tasking). Less is known about these other forms of MCI. Many feel that MCI is a precursor or a continuum of dementia and that it should be treated proactively, through prevention of cardiovascular risk factors, an active cardiovascular exercise program, and an active and social lifestyle. 
Localization of memory problem
The next step in evaluating a patient with memory loss is determining the localization of the memory loss. Memory has traditionally been divided into the following memory systems: episodic, working, semantic, and procedural memory.  Separate brain structures are responsible for each of these memory types.
Episodic memory: lasts minutes to years, and localizes to the hippocampus and limbic circuits. 
Working memory: a type of memory lasting seconds and involving active rehearsal of information; relies on the integrity of the prefrontal cortex, and the echoic memory in the angular gyrus. 
Semantic memory: typically consists of factual information (e.g., knowing what a certain object is and what it is used for, knowing who was the first President of the US), and localizes to the inferolateral temporal lobes. 
Procedural memory: pertains to driving a car or riding a bike, and involves the basal ganglia, cerebellum, and supplementary motor area. 
The type of memory impairment manifested through the history, physical exam, and neuropsychological testing can give an indication of the localization of the disease process.
Etiology of memory loss
The final consideration is the cause of the memory loss. Neurodegenerative, inflammatory/infectious, metabolic, vascular, traumatic, episodic, and endocrine processes can all produce memory impairment through compromise of the limbic and prefrontal circuits.