Solar energy storage has been an active research area among the various solar energy applications over the past few decades. As an important technology for solving the time-discrepancy problem of solar energy utilisation, seasonal/long-term storage is a challenging key technology for space heating and can significantly increase the solar fraction. It widens the use of solar collectors and results in better solar coverage of the space heating demand. This paper reviews all three available technologies for seasonal heat storage: sensible heat storage, latent heat storage and chemical storage. Sensible heat storage is a comparatively mature technology that has been implemented and evaluated in many large-scale demonstration plants. Water, rock-sort material and ground/soil are frequently used as storage materials. Latent heat and chemical storage have much higher energy storage densities than sensible storage, which means that they can remarkably reduce the storage volume, and they seldom suffer from heat loss problems. However, the latter two technologies are currently still in the stages of material investigations and lab-scale experiments. The characteristics of each concept have been presented in detail in this review. The latest studies and related projects are reviewed. The paper is structured as follows: fundamental investigation on storage materials, existing plants or projects and future outlook. (C) 2013 Elsevier Ltd. All rights reserved.