Walleye (Sander vitreus, formerly Stizostedion vitreum) is a freshwater perciform fish native to most of Canada and to the northern United States. It is a North American close relative of the European pikeperch. The walleye is sometimes also called the yellow walleye to distinguish it from the blue walleye, which is an extinct subspecies formerly found in the southern Great Lakes.

In some parts of its range, the walleye is known as the colored pike, yellow pike or pickerel (esp. in English-speaking Canada), although the fish is related neither to the pikes nor to the other pickerels, both of which are members of the family Esocidae.

Genetically, walleyes show a fair amount of variation across watersheds. In general, fish within a watershed are quite similar and are genetically distinct from those of nearby watersheds. The species has been artificially propagated for over a century and has been planted on top of existing populations or introduced into waters naturally devoid of the species, sometimes reducing the overall genetic distinctiveness of populations.

The common name, "walleye", comes from the fact that their eyes, like those of lions, reflect white light. This "eyeshine" is the result of a light-gathering layer in the eyes called thetapetum lucidum, which allows the fish to see well in low-light conditions. In fact, many anglers look for walleyes at night since this is when major feeding patterns occur. The fishes' eyes also allow them to see well in turbid waters (stained or rough, breaking waters), which gives them an advantage over their prey. Thus, walleye anglers will commonly look for days and locations where there is a good "walleye chop" (i.e., rough water). This excellent vision also allows the fish to populate the deeper regions in a lake, and they can often be found in deeper water, particularly during the warmest part of the summer.

Walleyes get their name from their huge marble like eyes, which feature a layer of reflective pigment. This gives the fish a distinctively walleyed look, but it also means that the walleye can see quite well in dim light. The most common prey of the walleye, the yellow perch, lacks this ability to see well in low light, so it is easy to see why the walleye has been such a successful predator. This well developed night vision also means that walleyes do a great deal of their feeding after dark.

Because their eyes are so sensitive to light, walleyes do not tolerate bright sunlight well. If they are swimming in clear water on a sunny day, walleyes are likely to go as deep as forty feet to avoid the harshest rays of sunlight.

Walleyes are not totally color blind, but they do lack the blue/yellow cells. For this reason, scientists believe that walleyes perceive the world entirely in shades of green and red. This means walleyes can see lures that are red, orange or green more readily than lures of other colors. It is important for the fisherman to realize, however, that the depth and clarity of the water can affect how the fish perceives the color. For this reason, many anglers will alter the color of their lures based on the depth of the water.

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