General Appearance
A graceful, alert, swift-moving little dog with saucy expression, compact, and with terrier like qualities of temperament.

Size, Proportion, and Substance
Weight - A well balanced little dog not to exceed 6 pounds.
Proportion - The body is off-square; hence, slightly longer when measured from point of shoulder to point of buttocks, than height at the withers. Somewhat shorter bodies are preferred in males. Disqualification - Any dog over 6 pounds in weight.

Head
A well rounded "apple dome" skull, with or without a molera. Expression - Saucy. Eyes - Full, but not protruding, balanced, set well apart - luminous dark, or luminous ruby. (Light eyes in blonde or white colored dogs permissible.) Ears - Large, erect type ears, held more upright when alert, but flaring at the sides at a 45 degree angle when in repose, giving breadth between the ears. Muzzle - Moderately short, slightly pointed. Cheeks and jaws lean. Nose - self-colored in blonde types or black. In moles, blues, and chocolates, they are self-colored. In blonde types, pink nose permissible. Bite - Level or scissors. Overshot or undershot bite, or any distortion of the bite or jaw, should be penalized as a serious default. Disqualifications - Broken down or cropped ears.  

Neck, Topline, Body
Neck- slightly arched, gracefully sloping into lean shoulders. Topline - Level. Body - Ribs rounded and well sprung (but not too much "barrel-shaped"). Tail - Moderately long, carried sickle either up or out, or in a loop over the back, with tip just touching the back. (Never tucked between legs.) Disqualifications - Cropped tail, bobtail.

Forequarters
Shoulders - Lean, sloping into a slightly broadening support above straight forelegs that set well under, giving a free play at the elbows. Shoulders should be well up, giving balance and soundness, sloping into a level back. (Never down or low.) This gives a chestiness, and strength of forequarters, yet not of the "Bulldog chest". Feet - A small, dainty foot with toes well split up but not spread, pads cushioned. (Neither the hare nor the cat foot). Pasterns - Fine.

Hindquarters
Muscular, with hocks well apart, neither out nor in, well let down, firm and sturdy. The feet are as in the front.

Coat
In the Smooth Coats, the coat should be of a soft texture, close and glossy. (Heavier coats with undercoats permissible.) Coat placed well over the body with ruff on the neck preferred and more scanty on head and ears. Hair on tail preferred to be furry. In Long Coats, the coat should be of a soft texture, either flat or slightly curly, with undercoat preferred. Ears - fringed. (Heavily fringed ears may be tipped slightly if due to the fringes and not to weak ear leather, never down.) Tail - Full and long (as a plume). Feathering on feet and legs, pants on hind legs and large ruff on the neck desired and preferred. Disqualifications - In Long Coats, too thin coat that resembles bareness.

Color
Any color - solid, marked or splashed.

Gait
The Chihuahua should move swiftly with a firm, sturdy action, with good reach in front equal to the drive from the rear, the hocks remain parallel to each other, and the foot fall of the rear legs follows directly behind that of the the forelegs. The legs, both front and rear, will tend to converge slightly toward a central line of gravity as speed increases. The side view shows good, strong drive in the rear and plenty of reach in the front, with head carried high. The topline should remain firm and the backline level as the dog moves.

Temperament
Alert, with terrier-like qualities.

Disqualifications
Any dog over 6 pounds in weight
Broken down or cropped ears
Cropped tail, bob-tail
In Long Coats, too thin coat that resembles bareness

source: http://www.akc.org/breeds/chihuahua/index.cfm