In a precast cladding panel system requiring no cast-in mounting hardware, each panel has a continuous notch along its upper edge, forming a ledge adjacent the panel's front face, and an upstand adjacent the panel's rear face. Each panel is mounted to a supporting structure using fasteners installed through openings formed in the upstand. The panels are configured such that their lower edges fit into the notches of underlying panels, with the front faces of all panels being substantially co-planar, aided by spacer means fastened to the supporting structure adjacent to the lowermost panel course. Air circulation behind the installed panel assembly is facilitated by spaced bumpers fastened to the supporting structure so as to space the panel upstands away therefrom. Alternatively, the panels may be mounted with the upstands directly against the supporting structure, with the panels having vertical air channels formed into the upstands and rear panels faces as appropriate.