Reconstruction of this Sienese painter's work, first proposed by Alberto Graziani in 1942 following a suggestion by Roberto Longhi, centers around a triptych representing the Virgin and Child between Saint Ambrose and Saint Jerome, dated 1436, in the basilica of San Bernardino all Osservanza in Siena. Before Graziani's proposal, this painting and the others now attributed to the anonymous master were usually assigned to Sassetta and considered the product of a Gothic interlude in Sassetta's artistic development. According to Longhi and Graziani, instead, the paintings in question belong to an autonomous figure who, besides drawing inspiration from Sassetta, also shows great interest in the work of some artists of the older generation, for example Giovanni da Milano and Masolino. In the two scholars' opinion, still widely accepted, the Master of Osservanza began his career sometime around 1430 with a Lamentation over the Dead Christ (now in the Monte dei Paschi collection in Siena), followed by a series of works that includes the Nativity of the Virgin in the Museo d Arte Sacra in Asciano, Stories from the Life of Saint Anthony Abbot divided between the NGA and other museums, the 1436 triptych, predella panels with stories from the Life of Christ (divided among the Fogg Art Museum in Cambridge, Massachusetts; the John G. Johnson Collection in the Philadelphia Museum of Art; and the Pinacoteca Vaticana), and the Saint George Slaying the Dragon in the church of San Cristoforo in Siena. The latest work in the painter's catalogue was thought to be a Gabella panel dated 1444 (Saint Michael Slaying the Devil; Archivio di Stato, Siena); however, the most recent criticism does not recognize this piece as belonging to the Master of Osservanza.
Not long after Graziani presented his hypothesis, cautiously suggesting that the artist might be identical with a Vico di Luca, documented together with Sassetta in 1442, an alternative was offered by Bernard Berenson and Cesare Brandi, according to whom the works grouped under the conventional name were executed at least in part by Sano di Pietro. Graziani had not missed noting the analogies, at times very strong, between the paintings assigned to the Master of Osservanza and those by Sano, but he felt along with numerous other scholars that the discrepancy in quality between their respective works was so great as to render their common authorship inexplicable. In more recent decades, however, several scholars have expressed more appreciative opinions of Sano's work, and it has also been observed that numerous motifs and compositional solutions used by Sano di Pietro derive from the repertoire of the Master of Osservanza. In other words, a continuity underlies the catalogues of these two artists, which creates great difficulties for anyone who tries to establish boundaries between them. Various proponents of the hypotheses concerning the Master of Osservanza hold, and not only recently, that the conventional name covers a collaboration among several artists, one of whom could be Sano di Pietro. Cecilia Alessi and Piero Scapecchi suggested that the anonymous artist was Francesco di Bartolomeo Alfei, a painter by whom no works are known today. But the number is also growing of those who, noting the gradual evolution in style in the Master of Osservanza's works from his Sassettesque beginnings toward the manner of Sano di Pietro; the documented relationship between Sano and Sassetta dating from Sano's youth; and the fact that no certain works by Sano are known before 1444, even though he was born in 1405 and was certainly already active in the 1420s, tend to accept an identification of the Master of Osservanza as the young Sano di Pietro. [This is the artist's biography published, or to be published, in the NGA Systematic Catalogue]
 See Longhi 1940, 188 note 26, and Graziani 1948, 75-87 (text of a lecture held in Florence in 1942); see Longhi 1948, 87-88.
 In his analysis of the Osservanza triptych, Pope-Hennessy, Sassetta 1939, 59-60, noted: "Instead of the strong colours and fiery flesh tones of the Duomo altarpiece [that is, the Madonna of the Snow, later acquired for the Contini Bonacossi collection and now in the Gallerie Fiorentine] the corollary of three-dimensionalism, we find pale pinks and blues, whites and thin golds, the luminous adjuncts of a more strictly ornamental purpose. The conclusion to which they lead us needs to be stated at no length. Between 1432 and 1436 Sassetta had become a Gothic painter."
 One of the reasons for which Graziani excluded the possibility that the Master of Osservanza could be Sano di Pietro was the existence of precisely this little panel in the Siena archives, dated in the same year as the Gesuati polyptych (Pinacoteca Nazionale, Siena), the earliest signed work by Sano di Pietro. But the panel is in fact quite distant from Sano's manner and at the same time irreconcilable with the group belonging to the Master of Osservanza, as was recognized by Brandi 1949, 199 note 61, and Pope-Hennessy 1956, 369-370, among others. Miklós Boskovits (author of the NGA systematic catalogue entries on the artist's work) believes that it is the work, albeit today very ruined and difficult to read, of Giovanni di Paolo, as van Marle, Development, 9 (1927): 416, had already thought; see Miklós Boskovits, "Iniziali miniate e tavolette di Biccherna...," ACr 73 (1985): 337.
 See Berenson 1909, Italian ed. (1946), 51-52, and Cesare Brandi, "Introduzione alla pittura senese del primo Quattrocento," La Rassegna d Italia 1, no. 9 (1946): 31, and (1949): 82-84, 199.
 A typical example of the harsh judgments expressed regarding Sano's work, and considered to justify the refusal of his identification as the Master of Osservanza, is that given by Laclotte 1960, 53: "un des plus vulgaires de la peinture italiennne, une sorte de Walt Disney siennois" ("one of the most vulgar in Italian painting, a sort of Sienese Walt Disney").
 For instance, Carli 1971, 26, and among the younger generation of scholars, Alessandro Angelini in Antichi Maestri Pittori: 18 opere dal 1350 al 1520 (exh. cat. Galleria Giancarlo Gallino, Turin, 1987), no. 13, and Keith Christiansen, "Sano di Pietro," in Painting in Renaissance Siena 1420-1500, ed. Keith Christiansen, Laurence B. Kanter, and Carl Brandon Strehlke (exh. cat. Metropolitan Museum of Art, New York, 1988), 138-139.
 Graziani 1948, 86, went so far as to speak of plagiarism concerning some motifs appearing in the predella panels of the Gesuati polyptych of 1444, which are now in the Louvre: "un plagio non privo di una sua tarda gentilezza, ma sempre un plagio, dei motivi del Maestro dell Osservanza" ("a plagiarism not without its own late elements of refinement, but still plagiarism, of the Master of Osservanza's motifs"). For other compositional ideas taken by Sano di Pietro from the Master of Osservanza, see Angelini in Antichi Maestri Pittori, no. 13, and Christiansen in Exh. cat. New York 1988, 135.
 Besides the Saint George altarpiece in the church of San Cristoforo in Siena, very often one or the other of the artists is credited with the Madonna from the Lehman collection in the Metropolitan Museum in New York, in which Christiansen in Exh. cat. New York 1988, 136, suspects a collaboration between them. Alessi and Scapecchi 1985, 26, augment the Osservanza Master's catalogue with two Madonnas generally recognized as Sano di Pietro's, one in the Lindenau-Museum in Altenburg (no. 74) and the other in the Brooklyn Museum in New York (no. 34.840), while Angelini in Antichi Maestri Pittori, no. 13, assigns to him the fragmentary Resurrection in the Wallraf-Richartz Museum in Cologne, also usually given to Sano.
 On the question of collaboration with regard to the scenes from the life of Saint Anthony in Washington and elsewhere, see the systematic catalogue entry (forthcoming late 2002/early 2003) for the NGA's panels. Christiansen detects collaboration in miniatures attributed to the Osservanza Master as well as in the Lehman Madonna (Metropolitan Museum, no. 1975.1.41) and in the Saint George in the church of San Cristoforo in Siena.
. See Alessi and Scapecchi 1985, 13-37.
 See "Sano di Pietro" in Dizionario enciclopedico Bolaffi dei pittori e degli incisori italiani. Dall'XI al XX secolo, 11 vols., Turin, 1972-1976: 10(1975): 142; Gemäldegalerie 1975, 389; Torriti 1977, 248-250, and, with further arguments, 1990, 178); Miklós Boskovits, "Il gotico senese rivisitato: proposte e commenti su una mostra," ACr 71 (1983): 267, and 1998, 294; De Marchi 1988, 298-302. On the responsibility of Sano for at least a part of the series, see also Christiansen, Siena 1990, 207-208.