Observation systems are increasingly used around the world for a variety of purposes; 2 critical purposes are to understand and to improve teaching. As observation systems differ considerably, individuals must decide what observation system to use. But the field does not have a common specification of an observation system, nor does it have systematic ways of thinking about how observation systems are similar and different. Given this reality and the renewed global interest in observation systems, this article first defines the observation system concept and then presents a framework through which to understand, categorize, and compare observation systems. We apply the framework to 4 well-known observation systems that vary in important ways. The article concludes with a discussion of the results of the application of the framework and some important implications of those findings.