Abstract: A case study of growth in retail trade and services in districts with socialist-time large housing estates in Leningrad–St. Petersburg during the period of 1989–2016 has revealed and validates the three principles of development of spatiotemporal systems (STS) in the process of transformation: change in the essence of transformation of an urban STS; continuity of spatial, temporal, and essential parameters of transformation of an urban STS; and the principle of possible misalignment of hierarchies of the spatial, temporal, and essential parameters of transformational urban STSs. Six stages of development of retail trade during the studied period are described: early transformation, the first stage of administrative reform, the stage of pavilions and markets, the stage of displacement of specific transformational forms, the stage of predominance of stationary trade forms, and the stage of corner stores. The corresponding spatial forms and particular features of the organization of retail STS are characterized by: the agglomeration of kiosks and vendors near metro stations and public transport stops; the agglomeration of kiosks and pavilions near metro stations plus markets; tents and mobile forms of trade near metro stations plus new markets; trade malls, stationary markets plus kiosk trade chains; hypermarkets, trade chains, temporary agglomerations near public transport stops and metro stations; neighborhood stores and illegal kiosks. A methodology for studying complex STSs is developed, and its effectiveness is fine-tuned and tested.