For Cats:

Ears:   Felines express a lot with their ears. In fact, it's one of the quickest ways you can tell    what kind of mood your kitty is in. If the ears are forward, they're generally happy or    relaxed. If they're airplaned out to the side or flat back, that's an upset cat! Pinned-   back ears mean a really, really upset cat - so ears are a good thing to watch.

Eyes:   Cats tend to be quite stoic, yet their eyes can be very reflective of their mood. When    the eyes are wide open and the cat is looking at you, she is listening to what you are    saying. But don't confuse this with a cat who is giving you a hard stare - eyes wide    and no blinking - which means 'Don't bother me; stay away!'. Blinking is common for    many cats; slow blinking in particular is a sign of happiness. On the other hand, if    your cat's pupils become dilated, that's a clear sign to back away. When a cat feels    provoked, the pupils get really big. You'll notice the eyes get glassy, and that's    because the reflective part in the back of the eye is getting bigger because the pupil    is dilated.   Whiskers:   With a cat, it's not so much the mouth that gives cues as it is the whiskers. When    they are forward and relaxed, that's a sure sign of contentment. When cats are    hunting prey, both the whiskers [wide and fanned out] and ears are forward. If your    cat is tense, excited or ready to attack, you'll see whiskers that are forward and    stretched out.

Tail:   While a dog's wagging tail is usually interpreted to be a sign of playfulness, it's the    complete opposite for a cat. If they are moving the tail from side to side, the faster    it's going, the more upset they are, it also matters how the tail is positioned. The    cat's tail is like a feline mood ring. Relaxed cats will walk along with their tails down,    and they will greet you with a tail that is up and not moving. If the tip of the tail    moves, it means that the cat is unhappy (e.g., 'Where is my dinner?'). A quick swish    or whip of the tail means the cat feels threatened.   Body posture:   While individual body parts are helpful, you have to look at the big picture to really    understand what a cat is thinking. Start with the whole body posture. In general, an    animal that is going to be offensive with you is going to try to make himself bigger.    That's how you get your classic Halloween cat: They arch their back, put their tail in    the air and make themselves puffy. That's a very upset cat. On the other hand, if    they try to appear smaller, the cat is feeling afraid. A cat will flatten down and drop its    ears to the side if it's trying to make itself as tiny as possible.

Sounds:   It may not be body language per se, but the sounds your cat makes speak volumes.    From purring to meowing, cats are very vocal animals. In fact, cats only meow to    humans. They will come and talk to you and tell you they want something, then you    have to figure out what it is they want. In addition to the more common sounds, cats    have also been known to growl, wail, and even chatter. Chatter is a predatory thing.    They do it with their jaws. Then there's the chirp, which is a friendly signal. When    you first come home, your cat will come over and you may get that little chirp. On the    flip side, hissing and spitting are unequivocal signs that your cat is feeling upset.

Behavior:   If you're still having trouble deciphering specific movements, look at your cat's    actions overall. The number-one indicator that something is wrong is a drastic    change in behavior. Cats are very routine-oriented... so if they start exhibiting things    like pooping outside the box, vocalizing or unprovoked aggression, it is a red flag    that something's not right. What about a happy cat? People tend to overlook one of    the happiest cues: kneading with the front paws, like they're making dough. That's    the sign of a really, really happy cat.

For Dogs:

Ears:   Because of variations among breeds' ears - some are pointy, others floppy - a dog's    ears are a little harder to read, but in general, the same principles for cats apply.    When your dog is relaxed, comfortable and under no stress, her ears will be held in    the natural position. When your dog is alert and watching something closely, her    ears will be raised and turned to whatever she is paying attention to. In addition, if    the ears are gently pulled back, it's a "sign of a friendly greeting." If the ears are    completely pulled down and back, however, your pooch is most likely feeling afraid.

Eyes:   People use the term "puppy-dog eyes" for good reason: A dog's eyes can practically    express the same emotional diversity as a human's. When a dog is stressed or    frightened, the eyes are not as wide open and they appear smaller. If the dog starts    to squint (assuming there is no sun in his eyes), it could be a sign he is in pain. Not    only will a dog change the size and shape of its eyes, but the direction of its gaze is    also a clear indicator of mood. Be forewarned. if a dog stares at you squarely in the    eyes, or avoids looking at you in a way that lets the whites of his eyes show, he's on    the defensive, so steer clear.

Mouth:   A dog baring its teeth is a universal sign of aggression, many people don't know that    a dog can also express other feelings with its mouth. When your dog is relaxed, its    mouth is usually closed or open just a small bit. Dogs who are stressed or afraid    often close their mouth and the lips are pulled back at the corners. To better    understand your dog's mood, it helps to factor in its whole body. One interesting    thing dogs do which can be mistaken for aggression is the submissive grin; that's    when they pull their lips back from their teeth, which might make you think the dog is    going to kill you. Put it in context with the rest of the body. If her posture is relaxed    and not stiff, she might be doing a submissive grin.

Tail:   While a wagging tail generally does mean that a dog is happy or excited, that's not    always the case. One of the greatest myths regarding canine body language has to    do with the dog's tail. The myth is that a wagging tail is the sign of a friendly dog. A    dog who is thinking about attacking may hold his tail high and move it back and    forth. The key is to look at the rest of the body; if you see the wagging tail with stiff    legs, tense muscles and lips that are starting to be pulled back, you could be in for    some trouble.

Body posture:   A dog's body posture tells a similar story to that of a cat. Aggressive dogs try to    make themselves look as big as possible. Their legs are stiff and they sometimes    rise up on their toes. Dogs that are afraid may lower their bodies, dropping to the    ground as if to say, 'It's OK, I'm so tiny and small, I'm not a threat'. The one posture    people seem to misunderstand the most is when dogs freeze. A dog who is panting    and then stops panting - that's a sign that something is about to happen. A freeze is    a sign that a dog is getting uncomfortable and it is often a threat. Signs a dog is    happy include an open mouth that looks relaxed and a shift in weight from side to    side. Another big cue is when their body is curved into a C shape, which is called a    "play bow."

Sounds:   If your dog's movements aren't telling you enough, listen to his bark. Barking can    indicate any number of things. You have to take it in context. Barking related to play    will usually be accompanied with a relaxed body posture and sometimes a wagging    tail, whereas barks that are short, insistent yips can mean 'Stop that!'. If you see a    stiff body along with a low-pitched growl, the barking can be a warning sign; also,    some dogs have barks that are intended to get your attention. Then there's    watchdog barking, which is a series of short and loud barks. It's a warning or alert    bark designed to let you know someone is coming - and to let [whoever that is] know    there is a dog here that is ready to handle the situation!

Behavior:   When all else fails, watch for significant behavioral changes. Destructive behavior,    like chewing things, is a big cue your dog is not happy. The number-one reason    they're destructive is they are not getting out enough or being exercised enough.    Another cue that something is amiss about their mental state is a drastic change in    behavior - sleeping in a different place, hiding more or sleeping more than usual.