I am married with no children and plan to leave everything to my husband. If he predeceases me, I'll leave my estate to certain charities. If he inherits my estate, I will want my former assets to go to those charities when he dies. Is that possible?
It is possible -- with some custom drafting by an experienced estate planning lawyer. Your wishes require special treatment because you are trying to control the details of what may happen many years in the future. Those conditions require careful planning.
A common arrangement -- especially with second marriages -- is to leave property to the surviving spouse in trust, and have it go directly to the intended beneficiary when the spouse dies. This arrangement doesn't give the spouse any discretion to redirect the property because it's all controlled by the trust you created. This arrangement will also require careful drafting, since you'll have to examine the estate and decide what happens if certain unexpected events arise between the time you die and your husband dies. For example, you might want to provide that your husband could use the assets (not just the income they produce) for a lifesaving operation.
In any case, you will probably need some good professional help to accomplish your goals. For some helpful hints about finding an attorney, see How to Find An Excellent Lawyer. Or use Nolo's Lawyer Directory to find and speak with an experienced estate planning attorney near you.