Supposedly both of the founders studied with the same jujutsu master, Sakeda (sp?). Both have an extensive array of joint manipulations as a result, which leads to similarity in some techniques. As for differences, Hapkido adds a great deal of kicks and punches, similar to those found in TKD, and they are emphasized more than the atemi (strikes) in Aikido, which while they exist, are often de-emphasized. In a good Hapkido school, there will also be a highly developed ground grappling program as well. Also, and this is a generalization, Aikido is more often positioned as a "purely defensive art" (i.e. an Aikidoka waits for his opponent to attack), but in Hapkido, initiating an attack is quite possible. In general this is true, but there are stylistic differences in each art, so some schools of Hapkido may be more "Aikido-like", and some schools of Aikido may look more like Hapkido. Both are excellent arts that a well-trained practitioner can use to fight in all ranges.