There exist marvelous and intricate adaptations in living things to perform complex functions. Take birds, for example. The feathers have a complex structure ideally adapted for the aerodynamics of flight. The bones are hollow to provide strength and light weight. An airplane created by human beings requires intelligent design to construct it and make it fly. The "intelligent design" position is that since an airplane requires intelligent design, the anatomy of a bird also must have been designed.
Intelligent design proponents believe that evolution can not explain the complexity of living things, and therefore it is merely a belief, regardless of the amount of scientific evidence. Evolution is based on two concepts: (1) Present life forms are descended from primitive ancestors, and (2) the process was guided by natural selection. In order to lend any credence to their theory of intelligent design, its supporters must establish that natural selection rules out any possibility of a designer. They have not done this.
The intelligent design theory is based on the assumption that natural selection requires that living things came into existence by mere chance alone. There is absolutely no scientific basis for this belief.
Intelligent design proponents use the fallacious argument that the "irreducible complexity" of functions in living things proves that they could not have evolved from previously non-existent structures. A bird cannot fly without hollow bones, avian wing structure, and feathers. Take away any of these components, and flight is impossible. Intelligent design theory is based on the assumption that all these components evolved simultaneously from a single set of mutations. According to ID, the mathematical probability that they all evolved simultaneously to produce flight is essentially zero. Therefore, according to ID theory, an intelligent deity must have purposely designed the bird.
ID supporters use the analogy of a mousetrap: take away any part and it ceases to function. It ceases to function as a mousetrap, but that doesn't mean that it has no function at all. Remove the latching wire, and it can still function as a memo holder or a tie clip. In the case of birds, fossil evidence indicates that a reptile mutation produced pinnate scales. Through a process of natural selection, the scales became feathers. After feathers evolved, front legs evolved to wing structures. Further down the line, natural selection favored birds with hollow bones. There is no basis for the ID "irreducible complexity" theory that they all would have had to evolve simultaneously.
In their presentations, intelligent design proponents stress that living things are perfectly designed for their purpose, with no potentially useful parts created by mutation. The evidence does not support this idea that mutations never create any useful modifications. It has been shown that bacteria that have components with no useful function can use that function to advantage under different environmental conditions.
Like their bedfellows the biblical creationists, intelligent design supporters present no evidence to support their position. Instead, they resort to negative attacks on evolutionary science based on distortions, misquotes, and outright falsehoods. Intelligent design theory is nothing more than the same old biblical creationism dressed up in scientific terms.
More information can be found in the Talk Origins faq on the flaws in intelligent design theory.