The present invention relates to transaction processing, for processing of payments between payer's (usually individual payers) and payee's (usually merchants. Conventionally, a payment transaction involves a user's account details being provided to a merchant device, e.g. by swiping a card in the card swipe of the merchant device. The merchant device then prepares a transaction message including information such as the user's account ID, merchant ID and payment information and forwards that message to a transaction processing system, which may comprise a transaction acquirer and an issuing bank. The transaction processing system approves the payment and returns confirmation to the merchant. In the present invention a device associated with the payer, which in a preferred embodiment is a suitably adapted mobile telephone, becomes involved in the payment transaction process. At one level, the transaction processing system requests from the payer electronic device confirmation that the transaction should proceed and the payer keys in an appropriate PIN to authorize the transaction. At another level, all the transaction processing information is provided from the payer electronic device to the transaction processing system and the transaction processing system or the payer electronic device then confirm that the transaction is authorized to the merchant device. This takes the burden of transaction processing off the merchant and also increases the security of the transaction as the payer is in control. In a further embodiment, the payer electronic device may also upload listings of products and select products at the same time as paying for them, the payee (merchant) being advised of the selected product.