The rich phase behavior of granular beds of bidisperse hard spherical particles in a rotating horizontal drum is studied by Discrete Element Method (DEM) simulations. Several flow regimes and various forms of radial segregation, as well as mixing, are observed by systematically varying the operational parameters of the drum, i.e. fill level and angular velocity, over a wide range. Steady states after several dozen revolutions are summarized in two bed behavior diagrams, showing strong correlations between flow regime and segregation pattern. An entropy method quantifies the overall degree of mixing, while density and velocity plots are used to analyze the local properties of the granular bed. The percolation mechanism may provide a qualitative explanation for the distinct segregation processes, and for the transient mixing in nonradially segregated beds. Initially blockwise segregated beds are found to mix before radial segregation sets in. High fill fractions (>65%) show the most intense segregation. (C) 2008 American Institute of Chemical Engineers AIChE J, 54: 3133-3146, 2008