First see whether they can be removed. In my rentals, there are very small diamond clips around the edges holding them into the frame. Once you find the "master" clip and push it in, all of the clips retract and the filter will release from the hood. On my home, the filter has little wires that you bend to release the filter. This enables you to not only clean both sides of the metal filter, but also inside the hood (where you will be shocked to see so much grease).
TayLee is correct about Krud Kutter, I soak my vents in a 3:1 ratio of krud kutter to water in the laundry tub so that the water is high enough to submerge the entire metal vent. Do not use a wire brush on the vents because it scratches the aluminum and can bend the fine wire mesh. A regular stiff dish brush works well to loosen the embedded grease. If your vents are aluminum like mine, do not use any product on them with bleach because it will permanently etch, discolor and pit the aluminum. (learned that lesson)
Inside the hood, I have found krud kutter straight is easiest because it cuts right through it faster. You have to constantly rinse out the grease from the sponge though because once it is full, you will just be spreading it around rather than removing it.
PS The only store carrying krud kutter in my area is Lowes on the cleaning aisle. It comes in a gallon container, can be diluted according to the type of job you are doing, it is biodegradable, non-toxic, fume free and safe for pets and children.