The previous chapters of this book have focused on the neural basis of memory in healthy adults. This chapter discusses five neurological diseases that affect the brain regions associated with explicit memory. Section 9.1 discusses patients with amnestic mild cognitive impairment. These patients have long-term memory deficits due to atrophy of medial temporal lobe regions including the hippocampus. Within a few years of being diagnosed with amnestic mild cognitive impairment, about half of these individuals are diagnosed with Alzheimer's disease, the topic of section 9.2. Patients with early Alzheimer's disease have more severe impairment of long-term memory and atrophy of the medial temporal lobe and the parietal lobe, two regions that have been associated with long-term memory (see Chapter 3). Alzheimer's disease patients also have abnormally high levels of proteins in the medial temporal lobe and the parietal lobe, which is thought to further disrupt processing in these regions. Section 9.3 focuses on patients with mild traumatic brain injury, who typically perform normally on working memory tasks but have increased fMRI activity within the dorsolateral prefrontal cortex and the parietal cortex, relative to healthy control participants. It is generally believed that such increases in fMRI activity reflect compensation, where these regions are recruited to perform normally on the task.