The biggest wealth transfer in history is about to happen — and it's now expected to be more than double what many thought it was. It's estimated that 45 million U.S. households will transfer $68 trillion in wealth over the next 25 years, according to Asher Cheses, a research analyst and lead author of a new report from financial services research firm Cerulli Associates.

Baby boomers, who hold the lion's share of that amount, are the wealthiest generation in American history — thanks in no small part to a 10-year bull market. That generation will pass down those assets over the next few decades. Here's what you should know to make sure that transfer is as smooth as possible.