They usually have a PitchCom receiver or, in some cases, a protective padded insert/cap in or under the hat. The receiver is part of the pitch- calling system, while some pitchers also use added padding for head protection.

What’s in the hat?

  • PitchCom receiver/earpiece setup. This lets the pitcher hear the catcher’s pitch call through a small wireless device.
  • Protective padding or special headwear. Some pitchers have used Kevlar or carbon-fiber style inserts, or hybrid cap-helmet designs, for extra protection.

Quick note

If you mean the inside of the hat for modern game use , the most common answer is the pitch-calling receiver. If you mean in general , some pitchers also wear protective inserts depending on preference and team approval.

TL;DR: MLB pitchers may have a PitchCom receiver in their hat, and some also use protective padding or specialty caps.