While there are some differences nutritionally, the advantages of fresh vs. frozen spinach pretty much balance each other out. It's super convenient to keep frozen spinach in your freezer and it remains "fresh" while refrigerated spinach tends to lose nutrients each day it spends in the fridge. To substitute fresh spinach for frozen, use one pound of washed fresh spinach for a 10 ounce package of frozen. Remove any large stems, chop roughly, and place in a large skillet. Heat over medium high heat, cooking and stirring for three to four minutes, or until the leaves are wilted. If your spinach was completely dry before adding it to the pan, add a tablespoon or two of water so the spinach doesn't burn. Cool and drain well, pressing out extra liquid.