“Bad banks” are back. The concept is simple. The bank divides its assets into two categories. Into the bad pile go the illiquid and risky securities that are the bane of the banking system, along with other troubled assets such as nonperforming loans. For good measure, the bank can toss in non-strategic assets from businesses it wants to exit, or assets it simply no longer wants to own as it seeks to lessen risk and deleverage the balance sheet. What are left are the good assets that represent the ongoing business of the core bank.

By segregating the two, the bank keeps the bad assets from contaminating the good. So long as the two are mixed, investors and counterparties are uncertain about the bank’s financial health and performance, impairing its ability to borrow, lend, trade, and raise capital. The bad-bank concept has been used with great success in the past and has today become a valuable solution for banks seeking shelter from the financial crisis.

But while the idea is simple, the practice is quite complicated. There are many organizational, structural, and financial trade-offs to consider. The effect of these choices on the bank’s liquidity, balance sheet, and profits can be difficult to predict, especially in the current crisis. Capital and funding markets are still fragile in many regions, and many asset classes have been severely affected.