Preparation - Baby bok choy often left whole or cut in half (or even quarters) lengthwise before cooking.  Cutting through the middle has the added advantage of making the thick center 'stem' section easier to cook.  Larger bok choy may be separated into individual leaves/stems, or chopped.

Cooking - Bok choy can be eaten raw or cooked using a variety of methods.

When cooking bok choy, you'll want to take two factors into account. First, you'll want to avoid over-cooking the leaves and the thin part of the stem. If the bok choy is chopped or in small pieces (e.g. in a stew or dumpling) this is less of an issue, but if it is being left more-or-less whole, you'll want to ensure that it doesn't get overcooked to the point of falling apart or losing its vibrant green colour. In general, shorter cooking times are better, and methods like blanching or boiling will often yield the best results.

The second thing to take into account is the thick portion near the base of the plant. This section takes longer to cook and has a stronger taste than the rest of the plant. Ideally, you want to cook this section until it is relatively transparent without overcooking the rest of the plant. Cutting small bok choy in half lengthwise makes it easier to cook this section, and often makes for better presentation.

While a number of cooking methods can yield excellent results, the balance between cooking the leaves and the stalks/base should be taken into account. Very high-heat methods like stir-frying often yield inconsistent results (the leaves over-cook before the stalks are finished). For this reason, many recipes recommend blanching or boiling bok choy before stir-frying. Alternately, if the stalks and leaves are chopped separately, the stalks can be added to a wok/pan first and cooked a little longer than the leaves. Braising tends to yield good results as well.

Any of the different bok choy varieties can generally be substituted for one another, so choose based on the size and colour you want for the final dish. The closely related vegetable Tatsoi (Rosette Bok Choy) can be generally be used in place of Bok Choy.

If you don't have any bok choy at all, you can substitute chard, though it tends to be a little more watery (and, depending on the variety, may contribute colour to your dish). Spinach leaves can substitute relatively well for the leafy green portion of bok choy. Among the Chinese vegetables, Chinese napa cabbage makes a decent substitute if you need a chopped/diced vegetable, and the thick stalks of both plants substitute well for each other in stir fry dishes.

Bok Choy diversity can seem a little daunting at first, but it's actually fairly easy to break down systematically.

First and foremost, let's differentiate it from Chinese (napa) cabbage, to which it's actually very closely related. The Chinese cabbage which forms large heads, often called napa cabbage, is a very close sibling of bok choy. Both are Brassica rapa - bok choy is the chinensis subspecies, while napa cabbage is the pekinensis subspecies.  Because they are so closely related, they do have flavour elements in common.  Nonetheless, the two are considered distinct, and look rather different from one another.  Because of nomenclature inconsistencies and translations, bok choy is sometimes referred to as Chinese cabbage, though this vague term is more often used to either a) broadly refer to Brassica rapa or b) refer to the head-forming Chinese napa cabbage.

Despite the great variation in sizes, there are really only two common type of bok choy.   Standard bok choy (shown above on the red background) has dark green leaves and contrasting creamy white stalks.  Shanghai bok choy (shown above on the light green plate) has bright green leaves and pale green stalks.

The two types are often further subdivided based on size.  Growers may grow certain cultivars best suited to one harvest size or another, but the basic principal underlying the variation seen on store shelves is simply harvest time; very small bok choy is young, larger bok choy is older.  In general, bok choy is sold in the following sizes (some or all of which may be available at any given store or at different points in the season).

Dwarf - very small bunches (usually around 7 cm/3 inches).  These extremely small bok choy may simply be labeled baby bok choy, and they can be used similarly, however they also make excellent salad greens.  Their very small size tends to make them sweeter and more tender - ideal for eating raw. Note that a similar vegetable (and member of the same species, Brassica rapa) called tatsoi or spinach mustard, is also sometimes called rosette bok choy.  It very much resembles a small, dwarf bok choy, though its growth habit makes it easier to harvest and sell as separate leaves, rather than small bunches.

Baby - small bunches (usually around 12 cm/5 inches or a little smaller).  Their small size makes them very popular as individual vegetable alongside main dishes or in soups, where one bunch (or half) makes an excellent serving size.  These are quite often labeled as baby bok choy.

Medium - an informal designation for larger bok choy bunches (usually around 18 cm/7 inches).  These tend to be a little large for a single individual, but are popular for separating out the individual leaves for use.  These are rarely labeled in any special way, and are generally just sold as bok choy.  Unlike large bok choy, they tend to retain the hourglass-waist of the smaller bunches.

Large - another informal designation, this time for very large bok choy (often as large as 30 cm/12 inches). These bunches tend to be stronger in flavour, and ideal of soups, stews, dumplings, and other dishes that call for a chopping. Unlike the smaller varieties, these large bok choy bunches often lack the characteristic hourglass waist of the smaller plants. If in doubt, look for the contrasting white stems and dark green leaves, or (in the case of Shanghai bok choy) the characteristic shape of the individual stems connected at a solid base.

As of 2019 there is no GMO bok choy in the food supply, however it is a monitored crop on a 'contamination watch list.' This means that bok choy is considered to be at some risk for accidental contamination through cross-breeding with GMO relatives that are in commercial production. All vegetables from this species (Brassica rapa) as well as several related plants in the mustard family are on this watch list, and are tested regularly by non-GMO watch groups.

Disclaimer - the information here is meant to act as a guide, and should not replace the advice of a trained medical professional.

Bok choy is an extremely healthy, nutrient-dense food, and it offers at least one benefit over some other leafy vegetables - it lacks the oxalate found in spinach, chard, and some other green vegetables. Oxalate (generally in the form of oxalic acid) is found in many foods, and is not in and of itself a problem; however it can bind calcium and, in doing so, become the principal component of kidney stones. People who suffer from kidney disorders, gout, rheumatoid arthritis, and certain types of vulvodynia are encouraged to avoid high levels of oxalate, making bok choy an excellent choice.

Bok choy, is a member of the family Brassicaceae (the cabbage and mustard family), a group often referred to as the cruciferous vegetables. Members of this family contains chemical components called glucosinolates, which are responsible for giving these plants their distinctive, pungent scent. These compounds have interesting effects in the human body. At normal levels (i.e. those achieved by consuming cruciferous vegetables as part of a balanced, vegetable-rich diet) they are believed to have some cancer-protective benefits. At unusually high levels (i.e. those achieved by eating extreme quantities of raw cruciferous vegetables, on the order of kilograms per day), glucosinolates can interfere with thyroid function, causing serious illness or even death. While there is at least one reported case of a person effectively overdosing on bok choy (an elderly diabetic woman who ate 1-1.5 kg of raw bok choy per day), it is worth noting that including bok choy in a normal, balanced diet is far more likely to have positive health benefits than negative ones. If you have a thyroid condition and you have any questions about eating bok choy or other cruciferous vegetables, consult with a medical professional.