代理理论就是委托代理理论，因为很多公司出现了经营权和所有权分离，所以就出现了委托代理的关系，也就是所有者将公司的经营权委托给了经营者。但这种委托关系存在一个缺陷就是存在信息不对称的问题，容易导致内部人控制问题，我认为信息不对称是委托代理关系的缺点和问题或者特征。（个人理解的观点，还是请楼主查一下专业文章）

The principal-agent model is based on the assumption of information asymmetry. Usually, we assume that the agent has more information than the principal has. The principal-agent model studies how to motivate the agent to work or to reveal his private information with the existence of the information asymmetry. For example, in a standard agency model, there is a moral hazard problem. The CEO's effort is unobservable to the shareholders. Therefore, the shareholders use performance measurements, such as accounting numbers, while designing the CEO's compensation so that the CEO is motivated to work hard.  

There may be adverse selection problem as well. For instance, a CEO privately learns the prospect of several potential projects and is going to make a investment decision. There may be some interest conflicts between the shareholders and the CEO. As a consequence, the CEO will not choose the best project for the shareholders to invest in. In this case, the shareholders can provide a list of contracts to the CEO and ask him to choose among them. The CEO's optimal choice will reveal his private information and makes the shareholders better off.
I would recommend you to read some contract theory books if you are interested in the principal-agent model and the information asymmetry.  

thanks。楼主这句换来源哪里？

Which sentence do you mean? I wrote it myself. You can find similar introduction about principal-agency model in any books on contract theory.
For instance, in Salanie's the economics of contracts 2nd ed, you will find the following statements in the introduction (Chapter 1.2):
Most of this book will use the Principal-Agent paradigm. There are two economic agents in this model: the informed party, whose information is relevant for the common welfare, and the uninformed party...

Without information asymmetry, there is no principal-agent problem because the first-best contract could be signed.

代理理论：由于现代的大型企业或是是上是公司多施行管理权与经营权的分离，股东就会将公司委托给管理者管理，形成了委托代理关系。代理理论主要涉及企业资源的提供者与资源的使用者之间的契约关系。这样对于同一个企业管理者和经营者的目的和掌握的信息不同，就会产生信息不对称。所以代理理论中一定会存在信息不对称。补充一下委托代理关系包括：股东与债权人之间的代理冲突，经理人员与股东之间的代理冲突，控股股东与中小股东之间的代理冲突。由于目的和掌握的资源和信息不同，所以以上三者之间一定会存在信息不对称。
信息不对称：信息不对称指交易中的各人拥有的信息不同。在社会政治、经济等活动中，一些成员拥有其他成员无法拥有的信息，由此造成信息的不对称。信息不对称可以包括多种的信息不对称，造成的原因可能有多种。代理理论的一个表现实信息不对称。
自己的理解和纯手打的，希望对楼主有帮组............