We develop a model of commodity money and use it to analyze the following two questions motivated by issues in monetary history:  What are the conditions under which Gresham's Law holds?  And, what are the mechanics of a debasement (lowering the metallic content of coins)?  The model contains light and heavy coins, imperfect information, and prices determined via bilateral bargaining.  There are equilibria with neither, both, or only one type of coin in circulation.  When both circulate, coins may trade by weight or by tale.  We discuss the extent to which Grsham's Law holds in the various cases.  Following a debasement, the quantty of reminting depends ont  the incentives offered by the sovereign. Equilibria exist  with positive seigniorage and a mixtdure of old and new coins in circulation