Even though online Bitcoin wallets are convenient as they can be accessed from anywhere in the world, there are several reasons not to store large amounts of Bitcoins in them. Each new Bitcoin address comes with a corresponding private key which is required to transfer coins on the former. In online wallets these private keys are often stored with the service provider, meaning that these providers are ultimately in control on your wallet. The latter has also made online services a popular target for hackers. There have been many exchange and online wallets that suffered from successful hack attacks in the past, but also more recent such as the exchanges Bitstamp and Bter in the first two months of 2015.