General signs of readiness
Essentials:

• The child is past the "unilateral No!" stage of the "terrible Twos" and is at least somewhat cooperative.
• S/he can understand and follow simple instructions and imitate simple tasks.
• S/he shows understanding of the terms "before" and "after" or "first" and "next" or past and present tense. ("Tell me before you have a BM," and "Did you already pee in your diaper?" have to have meaning to the child.)
• Bowel movements are regular and predictable.
• S/he understands, and can also say (or gesture for): potty, go, pee, poop (or the family's equivalent) and the body parts involved. The euphemism "going to the bathroom" will be interpreted literally by the young child as "entering that room," so be frank about the activity intended.
• No major changes in the child's life are happening or impending (birth of sibling, move to new home or daycare/preschool, etc.).

Helpfuls

• The child has begun to develop other self-help skills, such as undressing/dressing, wiping own hands and face after meals, handwashing, etc.
• shows by squirming, grimacing, running to a special corner, or pausing during play that s/he is about to eliminate. Always call the child's attention to what is happening: "You feel your pee or poop ready to come out," or, "Now it's in your diaper. Let's go change you."
• tells you that s/he is about to eliminate. Praise him/her for telling you - you certainly want this behavior to continue throughout training!
• dislikes the feeling of being wet or soiled. Disposable diapers tend to be comfortable even when wet, so consider switching to cloth if child is unwilling, although otherwise ready to train.
• can accurately tell you whether the diaper being worn is wet/dry, clean/soiled. Confirm and praise when the child assesses these sensations correctly.
• is embarrassed about wetting/soiling in diapers as "babyish," or wants to wear big-kid underwear like a sibling/peer/friend.
• is able to pull loose-fitting pants both down and up again.
• asks to use the potty or toilet.
• is willing to sit on potty or toilet and try.

More signs of physical readiness
A toddler becomes physically ready to toilet train when his bladder has increased its capacity to hold urine and his bowels need to move less frequently. Also, as the sphincter muscles develop, the child can learn to keep them closed until s/he reaches the potty and sits.

We can see if the child is physically capable of potty learning by noticing how long s/he keeps diapers clean and dry between changings: at least 2 hours dry at a time, and 3 or fewer BM's - and not runny - per day.

S/He also needs to be coordinated enough to run quickly to the bathroom and to pull loose clothing off and on again.

S/He needs to be physically comfortable eliminating. If s/he is straining or seems in pain while defecating, check with the doctor (consider dietary changes), and resolve the difficulty beforehand.

More signals of emotional readiness
Many toddlers develop a strong interest in cleanliness and order - often to the point of obsession, so that a sticky hand or a spot on a toy can no longer be tolerated for an instant. Potty training puts this tendency to good use.

The child may develop a sense of self, declaring him- or herself to be "not a baby" but a "big kid" and sedulously imitating the big people s/he admires. One of the clearest delineations between a grownup and a baby is toilet mastery, so that in many cases the child will be motivated to train both in order to move into the adult camp and for the satisfaction of mastering his body in a new way. This drive for independence remains after the negative/oppositional phase that children pass through at about age two and can be harnessed to encourage toilet learning.

Sometimes a younger toddler's initial interest in using the potty will not persist. At 18 months or so, children are curious about everything they encounter, whether or not they are ready to pursue mastery. Be ready to defer training if the younger tot with few readiness indicators loses interest.

Remember also that toileting is a time of intimate care of child by caregiver; a child who resists training may do so for fear of losing babyhood, of having to grow up "too fast." Offering independence in exchange for dependence (e.g., enlisting the child's help in donning or discarding diapers) and giving warm encouragement and congratulations along with frequent hugs and judiciously chosen rewards, can make acquiring this grownup behavior a triumph rather than a trial.

What Happens - Usually
The moment of readiness varies from child to child but generally falls between the ages of 20 months and four years. Boys usually train later than girls, but this is not always the case. Children who look up to older siblings may train more quickly than a firstborn. Bowel control is usually accomplished before full bladder control, and daytime control usually before nighttime control. Achieving complete dryness (day/night) may take days or months.