This invention relates to photonic biosensor arrays in particular employing plasmon resonance based sensing, and to methods and apparatus for reading such arrays. A biosensor array for plasmon resonance-based sensing of a plurality of different biological targets simultaneously, the array comprising a transparent substrate having a surface bearing a plurality of assay spots for plasmon resonance sensing, each of said assay spots comprising a discrete metallic island, a said metallic island comprising a plurality of metallic nanoparticles to which are attached functionalizing molecules for binding to a said biological target, different said islands bearing different said functionalizing molecules for binding to different ones of said biological targets, and wherein total internal reflection of light at said surface at a wavelength at or near a said plasmon resonance results in scattering of said light away from said surface, said scattering being modulated by said binding of said biological targets.