Strong plastics consist of two-phase arrangements of pairs of brittle solids which in combination may be very tough. Improvement now depends upon an understanding of brittle solid theory. The strength of brittle solids is generally very low but may occasionally approach the very high theoretical values. This is due to the high inherent strength being generally marred by tiny defects. When these defects are removed or contained high strength may be achieved. The various mechanisms by which this may be done are exemplified in mica, common salt, glass, metal whiskers and so on. The relevance of these substances and mechanisms to the manufacture of plastics is discussed.