First, a brief account is given of the principles underlying the use of the optical diffraction pattern of a perfectly helical particle to determine its helical lattice. The practical value and difficulties of this method are then revealed by a survey of its application to three different helical components of the T-even bacteriophage tail. The analysis of the structure of contracted sheath is much facilitated by the existence of two kinds of particle (contracted sheath and polysheath) with apparently the same helical lattice. Polysheath yields excellent optical diffraction patterns, and contracted sheath (which does not) reveals directly the number of helices in one set (information unobtainable from polysheath). The clear annuli of the extended sheath simplify the determination of all its structural parameters except for the number of helices in any set. For estimating this number, visual inspection is in one respect superior to optical diffraction, but an unambiguous solution has not hitherto been possible. The helical structure of the core is so faint that optical diffraction is indispensable for obtaining reliable structural data. Even with this method, only one structural parameter has been determined so far: the core is composed of annuli whose spacing is apparently identical with that of the annuli of the extended sheath.