Is knowledge true justified belief?

The idea that knowling something is the same as having a true justified belief has been around since Plato. However, in 1963 Edmund Gettier demonstrated that not all true jusfied beliefs count as knowledge. It is not true, hence, that knowing something is having a true justified belief about it. He did this by showing that there are cases in which we can have true justified beliefs but still not have knowledge. Since Gettier many authors have created new cases to demonstrate this. These cases are known collectively as 'Gettier-cases'. 

Here is a Gettier-case that shows that knowledge isn't true justified belief. Imagine you are looking at a field and you see what you believe is a sheep in the field (but is actually a wolf in sheeps clothing). Unbeknownst to you, there is a real sheep in the field with the wolf, but it is obscured from view by a hill. As the object you see looks like a sheep and is in a place where sheep normally are you are justified in believing it is a sheep. Furthermore, as there actually is a sheep in the field it is also true that there is a sheep in the field. It seems then that you have a justified, true belief that there is a sheep in the field but it would be strange to say that you know there is a sheep in the field. Therefore, whatever knowledge is, it is more than merely having justified true beliefs about it.