Persons with mild cognitive impairment (MCI) do not meet criteria for Alzheimer's disease (AD) but are at high risk for developing the disease. Presence of a memory deficit is a key component in the characterization of MCI. This chapter presents empirical studies that attempt to describe and understand the nature of the memory deficit in MCI with a focus on episodic memory and working memory. Cross-sectional studies report prominent deficits of episodic memory characterized by impaired encoding of the contextual information that makes up complex events. This results in reduced free and cued recall, impaired recognition, and impaired associative learning. Although semantic encoding is found to be impaired in conditions that rely on explicit and intentional retrieval, preserved semantic processing is found with automatic conditions of testing. Studies indicate the presence of a partial deficit of working memory with the ability to divide attention being most severely impaired. However, there appears to be heterogeneity as to the extent of the working memory impairment. The presence of vascular anomalies on MRI, as well as being in a more advanced stage in the continuum from MCI to AD, are associated with more severe and more pervasive working memory deficits. Finally, longitudinal studies indicate that the combination of episodic and working memory deficits represents a strong predictor of progression from MCI to AD.