The aim of this thesis is to give general answers to two questions: What is a good scientific concept? What is the nature of conceptual debates in science? In order to answer these questions, I will look at historical examples of conceptual change in science and at contemporary conceptual debates in science. Examples include the concepts of species, innateness, mass, biological individuality, planet, and homology. These examples will be used in a general discussion of what scientists and philosophers of science have put forward as virtues and vices of scientific concepts, and I will look at the controversies surrounding some of them. Some of the virtues and vices of concepts that will be discussed are inconsistency, conflation, simplicity, intuitiveness, theoretical significance (to support predictions and explanations), and applicability (to be easily applicable). Different resolutions that have been proposed to conceptual debates in science will also be discussed and their advantages and disadvantages will be compared. These include monism, pluralism and what I will call the unification resolution, which is about unifying the competing definitions by coming up with a general definition that somehow includes the competing definitions. I conclude by distinguishing between primary virtues like theoretical significance which are the most important virtues, secondary virtues like simplicity that are less important, and alleged virtues which are virtues that have been put forward as criteria for good scientific concepts that may not be virtues of concepts. I then show that there can be different levels of disagreement in conceptual debates in science, for example that scientists can disagree because there is disagreement about what a good scientific concept is. Finally, I argue that while the general approach pursued in this thesis is useful, it has some limitations because there might be very local or domain specific reasons to change concepts and many, perhaps even all, conceptual debates may therefore be unique.