Crema is the lovely layer of thickly effervescent foam that defines well-crafted espresso, an emulsion of the essential oils contained in the coffee. This presence of a thick layer of richly aromatic, reddish brown crema indicates that all culinary factors were met satisfactorily during the preparation of the shot. Espresso is all in the nose. The aroma of espresso lives in the crema so swirl it around. Get your nose right down in there. Inhale deeply.