This lecture covers the design of Infinite Impulse Response (IIR) filters, which were developed long before digital processing. It explains the conversion of analog designs into rational transfer functions, with the help of numerical packages like Matlab. The lecture details the design of Butterworth, Chebyshev, and Elliptic lowpass filters, discussing their magnitude responses, design parameters, and test values for order, cutoff frequency, and errors. Examples are provided for each filter type to illustrate the design process.