It is common knowledge that the practice of improvisation, variation, and ornamentation was widespread during the late Renaissance. Numerous authors testify to this, and scores of manuals exist that ''teach11 this particular skill. One of the most important theorists to write such a manual was Lodovico Zacconi, a monk of the order of Saint Augustine. Chapter LXVI of Part I of his Prattica di Musica is one of the most detailed of its kind. Because there is a genuine revival today of performance of Renaissance vocal music, and because sufficient knowledge is yet to be had of the practices of this period, it was felt that a translation of this chapter was advisable and necessary. Although writing in the loquacious, redundant, and flowery style so common to all Renaissance writers, Zacconi does go into much detail as to how ornaments and decorations should be inserted into already existing poly-phonic choruses. The advice he urges to be adopted can be summarized as a simple list of do's and dont's: pronounce your vowels and words distinctly; make only a few departures from the written line; insert as many notes as you can possibly manage but separate them, articulate them and, above all, stay in tempo; decorate the middle of a piece as well as the ending; repeat any note pattern as often as you please and feel free to transpose any example to fit the positions of the Guidonian hand; don't begin an ornament before the other voices have made their entrances; don't ornament a phrase while the other voices are silent; don't sing an ornamentation as a solo; and don't decorate every single syllable and word. The reader of Zacconi soon realizes that this practice of ornamentation, though much abused by many singers, was a highly sought after art that lured every one who aspired to be called a professional singer. One can observe that in a situation such as this, where the performer held such power over a composer, the immediate que8tion is one of taste and judgment. Consequently, the artist who practiced this skill was not only tested for his technical prowess but also for his good taste, moderation, and general aesthetic approach to music. By far the most rewarding feature of the chapter are the numerous musical examples (254 in number) which provide the reader with visual testimony as to exactly what sort of diminutions were employed during this period. While the nature of the examples is typical of what is known about Renaissance style, nevertheless these examples corroborate that knowledge. The only disappointing feature found in Zacconi is that he fails to clarify the very things the modern student wants to know most of all: what, where, how, and how many embellishments were to be made; and was the student to select the examples shown and perform them exactly as written, or were these meant to show him how to insert ornamentations in an "impromptu" manner as the need was felt? Since Zacconi, as in the case of all the other authors in this field, fails to enlighten us on these points, we can infer that the practice of improvisation must have been so universal, so taken for ~ranted that the writers did not think it necessary to spell out that which was so obvious to them. Finally, in an effort to put theory into practice, it was felt that a Renaissance polyphonic composition ought to be especially ornamented according to Zacconi's directions as a service to the reader And added as an Appendix to the thesis. Consequently, a sacred motet, Da pacem domine, by Orlando d' Lassus, a contemporary of Zacconi, was chosen. It is hoped that this first attempt by this writer will prove satisfactory in helping the practicing musician of today to understand better the probable nature of the vocal music of "the golden age of polyphony."