 So, today we want to start thinking a little bit about animal tissues. And as we start thinking about animal tissues, I want you to remember when we were studying levels of organization, which is a very important thing throughout all of biology. If you just remember back to levels of organization, we start out with the atom, and then we go to the molecule and the macromolecule, all of which you've studied. We then think about the cell and all the different kinds of cells. Then when we have two or more cells that work together to perform a particular function, we have a tissue. And then the next level, above the tissue level, is the organ level. And of course, when we have two or more tissues working together to perform a particular function, we have an organ. And then two or more organs working together to perform a particular function gives us an organ system. So, when you're studying animal anatomy and physiology or human anatomy and physiology, in this case, it's really important to always think back to levels of organization and just take note of where you are in this hierarchy of structure. And so, when we talk about animal tissues, we'll be looking at the cells that the tissues are made of, but you also want to think forward to the organs, and we'll talk about some organs in particular and how the different tissues come together to make an organ and then what the purpose of that particular organ is. Now, in animal tissues, there are four categories. So there are four broad categories of animal tissues, and then within each category, there are specific tissues that you need to know about. Now, there are all kinds of different tissues, but we're going to look at the basic tissues that are found in your body, for example. So, the four categories of tissues include epithelial tissue, connective tissue, muscle tissue, and nervous tissue. So, that's simple enough. Just remember, there are four different categories, epithelial, connective, muscle, and nervous. Each of these tissues plays a particular role within a given organ and within the organism.