Most houseplants are tropical in origin, so bring them indoors long before frost arrives. They can suffer injury at temperatures in the 40s. In addition, it's quite a shock to them to be moved from chilly outdoor nights to a toasty warm room indoors. They'll suffer less stress if you make their move more gradual. If they've been in full sun, move them to a shady spot for a week or so before taking them to the lower light levels inside your home. Some of the leaves on your plants may turn yellow and fall off soon after the transition. This is normal. They're simply adjusting to the lower light levels indoors. As a precaution, give them a shower to wash off pests and dust accumulated outdoors. Then inspect the plants for insects before bringing them inside. Control any pests with insecticidal soap, horticultural oil, or other appropriate pesticide. For a couple weeks keep the vacationing plants isolated from those that were left indoors all summer, until you're certain they have no other unwanted hitchhikers.