Search all chats
You remember a detail from an older chat but not which conversation it was in, so once your history reaches a few months, scrolling the sidebar stops working. Search all chats looks through every chat title and every message.
On a computer

Search all chats sits at the top of the sidebar, so click it there. You can also press Ctrl+K, or ⌘K on a Mac, to open the same window.
Type a word or a phrase, and matches show up under two sections: Chat titles come first, then Messages below. Each match is highlighted, so you can easily tell which match is the one you are looking for.
When you click a chat title, it opens that chat. When you click a message, it opens the chat at exactly that message.
Narrow the results
The row under the search field lets you narrow a long list, so you can find what you want faster. Discreet and Shared show only chats with those settings, which helps when you remember whether the chat was discreet or shared. Date opens a calendar where you can pick a single day or a range. Once you turn a filter on, it stays active while you change the search.
If you open the search without typing anything, it shows your recent chats instead. So the same window also works as a quick overview of recent conversations.
On a phone
On a phone, open the sidebar from the menu button at the top left and tap Search all chats. Tap the × when you are done to close the search and return to the chat.
Once a chat is open, Search a conversation finds a word inside it.