Caterpillars and aphids are two common leaf-eating pests that can munch away at your greens before you have a chance to harvest them, and as an organic gardener, they are the two things I search for and destroy immediately. Luckily, there are a few ways to control them (vegans, close your eyes!). Here's where you start: carefully scan the front and back of your leafy greens as well as the center of the plant stems for any pests.
An indicator of caterpillars, is of course, giant holes found in the center or exterior part of the leaves. You should also check the center of the plant stem for caterpillar eggs, which sort of look like rehydrated chia seeds. Eggs should be sprayed off using a hose, and as for the caterpillars, I feed them as well as the hole-y leaves to my chickens (FYI, chickens are natural omnivores). If you don't have access to chickens (as much as I hate to say it), simply stomp on the caterpillars.
Aphids usually appear on the backside of leafy greens but they can sometimes appear on the base of the plant. The eggs aren't gelatineous but rather are hard, black, and practically glued onto the leaves. The aphids themselves look tiny and green. I usually pick of the affected leaves and feed them to my chickens (seriously, chickens love your garden's pests!). Otherwise, simply pick and discard the affected leaves, then rub some crushed oyster shell on the remaining plant leaves and center of the stem. To aphids, it's like walking across broken glass, and they will avoid those leaves at all cost.
Sorry if this made you squeamish, but controling pests like these is how you can end up with beautiful, lush leafy greens (like the organic spinach from my garden pictured above). Trust me, it's worth it.