  Here we describe the design and performance of the Spider instrument. Spider
is a balloon-borne cosmic microwave background polarization imager that will
map part of the sky at 90, 145, and 280 GHz with sub-degree resolution and high
sensitivity. This paper discusses the general design principles of the
instrument inserts, mechanical structures, optics, focal plane architecture,
thermal architecture, and magnetic shielding of the TES sensors and SQUID
multiplexer. We also describe the optical, noise, and magnetic shielding
performance of the 145 GHz prototype instrument insert.
