Each contractual party must be a "competent person" having legal capacity. The parties may be natural persons ("individuals") or juristic persons (" corporations "). An agreement is formed when an "offer" is accepted. The parties must have an intention to be legally bound ; and to be valid, the agreement must have both proper "form" and a lawful object. In England (and in jurisdictions using English contract principles), the parties must also exchange " consideration " to create a "mutuality of obligation," as in Simpkins v Pays . 
The assignee steps into the shoes of the assignor, but this doesn't mean that the assignor then walks away with no obligations or liabilities. An assignment doesn't always completely relieve the assignor. Some contracts include an assurance that the original parties guarantee contract performance. This means that the original parties will fulfill the terms of the contract. So, if the assignee fails to perform the contract, the assignor must step back in and do so. Also, remember that an assignment can be only an assignment of rights. In that case, the assignor retains the obligations of the contract even though the assignee reaps the benefits.