This book presents a selection of works on pattern formation and stability of magnetic colloids. Magnetic liquids can be investigated in different scenarios. Geometry (quasi 1, 2 and 3 dimensional vessels ), scales (molecules, macroscopic particles) and the type of suspension (e.g., ferromagnetic, superparamagnetic) employed in experiments completely modify the aggregation process. The observed patterns in the fluid range from surface waves to bulk chains and bundles. The approaches presented in this book use standard statistical means such as the Gibbs free energy and chemical potential. Numerical works are implemented employing methods such as Monte Carlo or Langevin dynamics simulations. Kinetic theory is used in theoretical approaches being successfully applied to algorithms such as the Lattice-Boltzmann method.