In this paper, we construct interior spike layer solutions for a class
of semilinear elliptic Neumann problems which arise as stationary sol
utions of Keller-Segel model in chemotaxis and also as limiting equati
ons for the Gierer-Meinhardt system in biological pattern formation. W
e also classify the locations of single interior peaks. We show exactl
y how the geometry of the domain affects the spike solutions.