IIR is a C++ library that implements fast IIR filters: bandpass, bandstop, lowpass, and highpass. These can be designed using popular analogue filter functions such as Butterworth, Bessel, Chebyshev, Elliptic, Legendre, and pole placement. Creation of a complete filter requires only a single command and filtering is performed with a single function taking a sample and returning its result instantly. The library is template-based and avoids new/mallocs throughout. This library has been adapted form Vinnie Falco's original work and has been converted to run on Unix systems and turned into a realtime causal filter.