Lipid droplets (LDs) have been the focus of intense research for the past decade because of their active engagement in lipid metabolism and relationship with diseases. In contrast to other intracellular organelles, LDs are composed of a mass of hydrophobic lipid esters that is covered with a phospholipid monolayer. The unique architecture makes the LD a formidable object to study by the methods available today, and many fundamental questions remain unanswered. This review focuses on some of those questions, such as how LDs form and grow, how proteins move to and from LDs, and how LDs are related to protein degradation; we will also discuss what is not known about LDs. We think that small LDs that have thus far eluded analysis are the key to resolving many of the above-mentioned questions.
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