Many retirees assume their highest tax years are behind them. For some, their biggest tax surprise is still ahead.

Retirement planning often focuses on a date or a dollar amount. But retirement itself is a major life transition. The most successful retirements aren’t just well funded. They’re well thought out.

It’s difficult to overstate how important Social Security benefits are to retirees in the United States. Consider the following: Social Security benefits typically replace about 40% of pre-retirement income for average earners. The system provides inflation-adjusted benefits to over 70 million Americans. 50% of the senior population rely on Social Security for at least half of their income.