The official currency in Laos is the kip (LAK). Officially, trading in foreign currency is not allowed, although some places in tourist areas still accept USD. Exchange money at a bank or licensed exchange counter, ask about fees first, and count your cash before leaving the counter.
Visa and Mastercard are accepted at many hotels and restaurants in larger towns, sometimes with a card surcharge. ATMs in Vientiane, Luang Prabang and Pakse often accept Visa and Mastercard, with limits of about 1 to 2 million kip per withdrawal (roughly 100 to 200 USD) and about 4 million kip per day depending on the bank. Fees may come from your own bank or from the ATM owner.
Small restaurants, markets and rural areas often take cash only, so withdraw enough while you are still in a larger city. QR payment is widely used locally but is usually tied to a Lao bank account, so foreign visitors should not rely on it as their main option.