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Bluetooth has the Secure Simple Pairing (SSP) feature, and Bluetooth Low Energy has the LE Secure Connections feature, which both use Elliptic Curve Diffie-Hellman Key Exchange and therefore is protected against passive eavesdropping. The standard also includes the ability to support authenticated pairing for protection against Man-in-the-Middle attacks; if you've ever had to see if a 6-digit number matches on the two devices that are being paired, then you are seeing the authenticated pairing taking place.

Even though the Bluetooth standard includes these features, many products don't actually use them and simply transmit data without any encryption or authentication procedure. This is particularly a problem with many Bluetooth LE products.