Minimum Description Length (MDL) is a model selection principle where the shortest description of the data is the best model. MDL methods learn through a data compression perspective and are sometimes described as mathematical applications of Occam's razor. The MDL principle can be extended to other forms of inductive inference and learning, for example to estimation and sequential prediction, without explicitly identifying a single model of the data. MDL has its origins mostly in information theory and has been further developed within the general fields of statistics, theoretical computer science and machine learning, and more narrowly computational learning theory. Historically, there are different, yet interrelated, usages of the definite noun phrase "the minimum description length principle" that vary in what is meant by description: Within Jorma Rissanen's theory of learning, a central concept of information theory, models are statistical hypotheses and descriptions are defined as universal codes. Rissanen's 1978 pragmatic first attempt to automatically derive short descriptions, relates to the Bayesian Information Criterion (BIC). Within Algorithmic Information Theory, where the description length of a data sequence is the length of the smallest program that outputs that data set. In this context, it is also known as 'idealized' MDL principle and it is closely related to Solomonoff's theory of inductive inference, which is that the best model of a data set is represented by its shortest self-extracting archive. Overview Selecting the minimum length description of the available data as the best model observes the principle identified as Occam's razor. Prior to the advent of computer programming, generating such descriptions was the intellectual labor of scientific theorists. It was far less formal than it has become in the computer age. If two scientists had a theoretic disagreement, they rarely could formally apply Occam's razor to choose between their theories. They would have different data sets and possibly different descriptive languages. Nevertheless, science advanced as Occam's razor was an informal guide in deciding which model was best. With the advent of formal languages and computer programming Occam's razor was mathematically defined. Models of a given set of observations, encoded as bits of data, could be created in the form of computer programs that output that data. Occam's razor could then formally select the shortest program, measured in bits of this algorithmic information, as the best model. To avoid confusion, note that there is nothing in the MDL principle that implies a machine produced