Small and slender and somewhat resembling fingers (hence the name), fingerlings are a low starch potato that holds its shape well making them a good, albeit petite, all-purpose potato. Though they come in a variety of colors (tan and red as well as purple), all have a thin skin and a denser flesh than round potatoes. Fingerling varieties include Russian Banana, Butterfinger, and Rose Finn Apple (which has rose-pink skin and yellow flesh that's blushed with red), Ruby Crescent, and Purple Peruvian.
Substitute any medium-starch potato, such as Yukon golds, especially baby ones.
Potatoes should feel heavy and firm, never soft, wrinkled, or blemished. Try not to buy potatoes in plastic bags since it's hard to evaluate them. Avoid those that are tinged with green. The green indicates the presence of solanine, which is produced when potatoes are exposed to light, either in the field or after harvest. This mildly poisonous alkaloid has a bitter flavor that can cause an upset stomach. If your potatoes turn green after you get them home, peel off all traces of the colored flesh before cooking.
Rinse the potatoes well and drain. Most recipes calling for fingerlings have you leaving the tender skins on, a good thing since their tiny size makes them tedious to peel. Store fingerlings away from light in a cool (not cold) and dry spot.
Smaller potatoes don't keep as long as their larger counterparts, and since they can be pricey, do use them before they sprout.