Sano was one of the most prolific and successful Sienese painters, the head of a workshop that satisfied the demands of civic and religious institutions in the city as well as those of private devotion. His production, technically always of a very fine quality, rich in decorative effects, and characterized by a brilliant palette, often contains motifs that appear monotonous and repetitive. Although in 1428 he was already listed in the guild of Sienese painters, his work is well documented in its various stages only from 1444 (Gesuati polyptych) until his year of death (Pietà, Monte dei Paschi collection in Siena, 1481), whereas the question of his early activity is still open for discussion.
Esteemed in the nineteenth century literature as a sort of Fra Angelico of Sienese painting and considered by some a typical representative of Sienese quattrocento mysticism, Sano is much less appreciated by twentieth-century criticism. In 1931 Bernard Berenson called him "the most monotonous, the most spiritless, the most vapid" of the city's artists, and almost all twentieth-century scholars feel that his production after the Gesuati polyptych of 1444 (Pinacoteca Nazionale, Siena, no. 246), which remains his earliest certain work, bears witness to a gradual but inexorable decline in his creative imagination and artistic capability.
In reality Sano's development in the decades around the mid-century, marked by numerous dated works, is in no way lacking in paintings of high quality. Particularly fine examples are his miniatures, notably those done for the Opera della Metropolitana in Siena from 1445, for the monks of Monte Oliveto Maggiore between 1459 and 1463, for the Spedale della Scala in Siena, and for the duomo of Pienza around 1462. But also in other kinds of painting--a fresco for the office of the Biccherna in Siena; in 1448 the predella of the altarpiece for the chapel in Palazzo Pubblico; in 1449 the Scrofiano polyptych (Sinalunga; now in the Pinacoteca in Siena, no. 255); in 1456 the Pope Callixtus III (Pinacoteca, Siena, no. 241); in 1458 the polyptych in San Giorgio at Montemerano (Grosseto); in 1462 the altarpiece for the duomo of Pienza--the artist reveals gifts of narrative fantasy and chromatic freshness, although his manner remains deeply rooted in the habits and tastes of early Renaissance painting in Siena and appears little inclined toward renewal.
Along with works on a monumental scale, he produced numerous small panels, in response to the religious needs of a wide audience, and particularly to the spiritual tendencies represented in the Franciscan observance movement promoted by Saint Bernardino and the Sienese order of the Gesuati.
The proposal to identify the works of his youth as those gathered under the conventional name of the Master of the Osservanza, which, having the triptych dated 1436 in the Basilica dell'Osservanza in Siena as their point of departure, cover the period from the third decade of the century to 1444, has been rejected by many scholars, who prefer to consider the author of the 1436 triptych and works with similar characteristics an artist quite distinct from Sano, mainly because of their generally very fine quality. In 2002, however, Miklós Boskovits (author of the NGA systematic catalogue entries associated with the artist) is inclined, along with several other scholars, to accept the hypothesis first proposed by Berenson and Brandi identifying the Master of the Osservanza as the young Sano. [This is the artist's biography published in the NGA Systematic Catalogue]
 Auguste F. Rio, De l'art chrétien, 4 vols., Paris, 1861: 1:98: "Sano... on pourrait appeler le fra Angelico de l'école siennoise" ("Sano... could be called the Fra Angelico of the Sienese school").
 Gaillard 1923, 184: "L'art de Sano di Pietro est essentiellement représentatif de la mystique siennoise du XVe siècle" ("the art of Sano di Pietro is essentially representative of fifteenth-century Sienese mysticism").
 Berenson 1930, reprint (1969), 50.
 Torriti 1977, 256, observes, for example, in speaking of the 1444 polyptych, "[S]embra veramente incredibile come Sano, senza prima aver nulla creato, possa essere giunto d'improvviso... a questo fulgido capolavoro per poi immediatamente dopo iniziare un lento declino nelle sue cento e cento opere" ("[I]t seems truly incredible that Sano, without having created anything before, could suddenly have achieved... this splendid masterpiece and then immediately afterward begun a slow decline in his hundreds of works").
 See Berenson 1909, Italian ed. (1946), 51-52, and Cesare Brandi, "Introduzione alla pittura senese del primo Quattrocento," La Rassegna d'Italia 1 (1946): 31.
Gaillard, Emile. Un peintre siennois du XVe siècle. Sano di Pietro. Chambéry, 1923.
Trübner, Jörg. Die stilistische Entwickelung der Tafelbilder der Sano di Pietro (1405-1481). Strasbourg, 1925.