The digital transformation leads to massive amounts of heterogeneous data challenging traditional data warehouse solutions in enterprises. In order to exploit these complex data for competitive advantages, the data lake recently emerged as a concept for more flexible and powerful data analytics. However, existing literature on data lakes is rather vague and incomplete, and the various realization approaches that have been proposed neither cover all aspects of data lakes nor do they provide a comprehensive design and realization strategy. Hence, enterprises face multiple challenges when building data lakes. To address these shortcomings, we investigate existing data lake literature and discuss various design and realization aspects for data lakes, such as governance or data models. Based on these insights, we identify challenges and research gaps concerning (1) data lake architecture, (2) data lake governance, and (3) a comprehensive strategy to realize data lakes. These challenges still need to be addressed to successfully leverage the data lake in practice.