The selection of a filter is based on a knowledge of engine requirements and filter performance. This paper describes some of the newer concepts in filtration theory, particularly as it relates to lubricating oil filters. Data are included to show that filter “life” should be based on the ability to prevent engine wear rather than pressure drop. These data also show the importance of oil quality. Comparisons are made between filter papers having different micron ratings, after it is shown that micron ratings can be misleading unless they are clearly defined. Comparisons are also made between surface type and depth type filters, indicating that it is what the engine needs that really governs filter selection.