When I first started programming I worked on a Banking software app with double accounting... 1) each account (cash, credit) is treated the same way 2) each transaction has 2 parts (adding to one account subtracting from another)==> money cannot "just appear/disappear" ie you receive money from a client (subtract from client account add to "Sales Account") 3) each transaction is recorded in a transaction log (minimum "From account", "To Account", "Amount",(we rcorded much more)). 4) over night the transactions in the log are actually posted to the relevant accounts and the log is cleared when i left said company i vowed i would never work on something like it again, the support for ensuring evrything was/is posted correctly - nightmare I have found that there are plenty of purpose written packages on the NET, some more adaptable that others. SAGE has a pretty good API - though you have to pay >£1k to get their developer licence- well worth it if u cost your time in writing it from scratch