This chapter targets the synthesis and the design of filters using nonuniformly sampled data for use in asynchronous systems. Data is sampled with a level crossing technique. It presents several filtering strategies, namely Finite Impulse Response (FIR) and Infinite Impulse Response (IIR) filters. The synthesis of filters in the frequency domain based on a level-crossing sampling scheme of the transfer function is also presented. Finally, an architecture for an hardware implementation of a FIR filter is given. This implementation is based on an asynchronous data-driven logic in order to benefit from the events produced by the asynchronous analog-to-digital converter.