The best defense against pests and diseases is growing healthy plants through sound culture and proper sanitation.  But even so, once in a while you may have to deal with pests.
Fortunately not too many pests will affect your orchids, but some of them will take determination to get rid of.  The insects that may affect your orchids are mostly mealybugs, scale, aphids, fungus gnats, spider mites, thrips and slugs / snails.
Check your plants after summering them outdoors
Before bringing your plants indoors again, check the leaves, under the leaves, around flower spikes, behind flowers and just under the rim of the pot for any sign of pests.
Chewed up leaves are a sure sign of damage from slugs or snails.
If you have summered your plants outdoors, even if you do not see any sign of insects you should consider treating them against insects before bringing them in.
Check your plants periodically
It is a good idea to check your plants periodically for any sign of insects.  It is much easier to eliminate insects before they severally infest your plants.
The two most common scales that may affect your orchids are the soft scale and the armored scale.
Soft scale may hide in dried sheaths at the base of pseudobulbs of Cattleyas or similar plants.  Remove the dried up sheaths to eliminate hiding places for them. Remove as many of the scale as you can see, then treat with an insecticide.
Hard scale looks like tiny turtles. They will usually be under the leaves of Phalaenopsis or Cattleyas, sometimes hiding in the pot. Remove as many as you can see, then treat with an insecticide.
They are white and look sort of cottony, may be 1/4” in size. They can be on or under the leaves, on flower stems, on buds, behind flowers, in the pot,....  Remove as many as you can see, then treat with an insecticide.
Removing as many of the above insects as you can.
For all of the above use a soft toothbrush or cotton swabs to remove as many of the insects as you can. Dip your toothbrush / your cotton swabs in alcohol before using them to remove the insects.
The mots persistent of them, as they reproduce on a short, 3 day cycle. Usually found on new growth, new leaves, on flower stems and flower buds, they suck the juices out of the plants and can cause substantial damage and leave marks on the leaves.  Because they fly it is difficult to remove them individually.  In warm sunny weather take the plant outside and use a garden hose to shake them off the plant.  Then treat with an insecticide.
Fungus gnats
Look like small black flies. Are mostly hiding in the pot and fly our when you water. They are attracted by potting material that stays damp and by decaying plant material (dead roots, leaves,...).  They feed mostly on dead plant material but they may attack roots, especially those of Cymbidiums. Because they hide in the pot you can not remove any. You have to treat them by immersing the pot in an insecticide solution.
Spider mite
Are very small and can not be seen individually without a magnifying lens. You can detect them by looking under the leaves for tiny silvery pits where they have sucked the plant juices. Another way to see if they are any is to place a white paper towel under the leaf and rub the leaf to make them fall on the paper towel. Spider mites will thrive in dry (too low humidity) environments.  The best defense is maintaining reasonable humidity, but if you have spider mites you’ll have to treat them with an insecticide / miticide. For best results immerse the pot in an insecticide / miticide solution.
They do not occur frequently, but if they do, you’ll notice it because of deformed or spotted flowers. They are difficult to eradicate because they tend to lodge in the flower buds and under sheaths where they are protected from insecticide sprays.
Caution - Warning
If you are going to use commercially available pesticides please always follow the directions on the label.  Some of these insecticides are very potent and you should make sure to apply all required precautions against poisoning yourself, others around you or your pets.
You can prepare a safe, effective insecticidal soap by mixing 1 teaspoon of a mild liquid dishwashing detergent (the 409 cleaner, regular, works very well) to a quart of lukewarm water.
How insecticides work
Systematic insecticides are to some degree absorbed by the plant and may offer residual protection for a few weeks.  Systematic insecticides tend to be more dangerous than other insecticides.
Many insecticides kill only the adult insects, not necessarily the eggs or the larvae (immature insects).
Insects develop resistance to insecticides.  What this means is that some of them are not affected by the insecticide and these will reproduce.  Treating these with the same insecticide will not kill them.
To avoid resistance you should consider rotating insecticides, that is you make the first application with one insecticide, the second application with another and the third one either with the first insecticide or with a third one. Rotating is not necessary with the insecticidal soap you prepare because this insecticidal soap works by suffocating the insects.  
Treating for a limited infestation
If the infestation is not excessive, spray thoroughly the new growths, leaves (both sides), flower stem, back of buds and flowers with the insecticidal solution.
Treating severe infestations
If the infestation is widespread dip the hole plant for 15 minutes of so in the insecticide solution.
For the treatment to be effective you have to treat the plant (spraying or immersing) 3 times, at intervals of one week (intervals of  3 - 4 days for aphids).
The reason you need to make more than one application is because the insecticide will kill the adults and a few days later the eggs will hatch and the cycle restarts unless you treat again to kill the hatches.  Most of the time three applications one week apart (3 - 4 days apart for aphids) will eliminate the insect population.
Caution !
If you are going to immerse plants in a solution :
• do it only on sunny days; if the weather is cool, cloudy or rainy, you’ll be much better off waiting a day or two before treating your plant(s),
• do it early in the day;  this will allow any water that got in between leaves or new growth to evaporate before nightfall,
• do it with room temperature or lukewarm water as a difference of 10 degrees or more between the temperature of the water and the room temperature may cause injuries to the plants.
These are tough to treat as they emerge at night.  In the old days the products to treat them were base on formaldehyde, a noxious product.  Today you can treat them with a very effective and safe product called sloggo, available either in granular or liquid form.
The product is available from Monteray Lawn & Garden Products, Fresno, CA (www.; Tel. 559-499-2100).
Occasionally you may come across a plant that has a virus.  This may manifest itself by concentric or elongated black or brown or discolored circles on the leaves or black streaks on flowers and leaves.  These will be repeated on all leaves / flowers.  New leaves / flowers will at first appear free of it but as they age the virus will manifest itself.  Unfortunately there is nothing you can do but discard the plant.
Bacterial and fungal diseases
These will appear if water stays in between leaves or if the potting material stays soggy, especially when the night temperatures are cooler (fall, winter, spring).
You can treat these with fungicides like RD 20 or Physan 27, but the best way is to avoid these problems by practicing proper culture.