The hearing impaired often have great difficulty understanding speech in surroundings with background noise or reverberation. A directional hearing aid might be beneficial in reducing background noise in relation to the desired speech signal. To this end microphone systems were developed with strongly directional characteristics, using array techniques. Considerable attention was paid to optimization and stability. Free-field simulations of several robust models showed that a directivity index of 9 dB can be obtained. Simulations were verified with a laboratory model. Based on simulations and measurements, two portable prototypes were developed and tested using a KEMAR-manikin. The KEMAR-measurements showed that the two prototypes gave an improvement of the signal to noise ratio of 7 dB in a fully diffuse sound field. The benefit of these microphone arrays for the hearing impaired was tested in a sound insulated room. One loudspeaker was placed in front of the listener simulating the partner in a discussion, and a diffuse background noise was produced by eight loudspeakers placed on the corners of a cube. The hearing impaired subject was seated in the centre of the cube. The speech-reception threshold in noise for simple Dutch sentences was determined with a normal single omni-directional microphone and with one of the prototypes. The results of the listening tests with 45 hearing impaired subjects showed an average improvement of the S/N-ratio of 7.0 dB for monaural fitting.