As the world populatian aging, hearing impairments become a high incidence chronic disease. Hearing aids is one of the most effective means of hearing intervention and hearing rehabilitation for presbycusis hearing patients. Various techniques of hearing aids have advanced significantly over the past decades, primarily thanks to the maturing of signal processing technology and electronic technology. Among these technologies, sound classfication, filter decomposition, noise suppression and echo cancellation are four basic algorithms for hearing aids. Based on deep understanding, we elaborate the algorithms in terms of aspects: the basic principles, the current research status, features and problems. In addition, by analyzing the current problems of hearing aids, three new research direction, auditory bionics, auditory cognition and selffitting hearing aids, are outlooked and briefly introduced.