An analysis of the nature of the air-borne particles which constitute the main source of contamination on outdoor porcelain insulators, and of the forces acting upon them in the electrostatic field around an insulator, suggests that these forces are inadequate to account completely for the observed distribution and the quantity of the deposits on insulator surfaces. Direct measurements of the field strengths close to the insulator confirm that the voltage gradients do not reach very high values under normal conditions, and that the electrical forces are proportionately weak. Investigations of the air flow over the insulators, and comparisons of the formations of air eddies observed in the flow over the geometrical forms associated with the complex shapes of insulators, show that the aerodynamic conditions have a considerable bearing upon the patterns formed by the deposits on the insulators. Wind-tunnel experiments provide results which illustrate the influence of the Reynolds' number, the applied voltage, the surface roughness, the wind velocity and the humidity, upon the distribution and amount of deposited matter. Moisture films constitute a special case of temporary contamination with sparking following the receding edges of the drying films. Finally, a method is suggested of estimating the flashover values of insulators from a consideration of the distribution of surface resistance as modified by the presence of deposited matter.