1 ForewordIn 1945, Vannevar Bush described an imaginary device,which he called Memex in his famous article called ‘‘As WeMay Think’’ [13]. He envisioned the device to be able torecord and link books read, microfilms watched and otherpersonal archives. Today, in the era of digital technology,Bush’s visionary article can be seen as the first efforttoward establishing the field of life logging. A system ortool that can digitally sense people’s state and contextualinformation in a continuous manner and record this infor-mation for long-term access is therefore often referred to aslife-log tool.Life-logs can benefit users in many ways. For instance,it has been shown that life-logs augment the user’s memory[12], and there are tools developed for this purpose [8].Life-logs can further be used to record information aboutoneself, which can benefit people with learning [1, 2],psychological studies [15], user modeling and personali-zation [4, 6] , social studies [3], historical studies, storytelling [4, 11] and health monitoring [5, 9, 10]. To date,there are only few life-log tools with very restrictedfeatures available on the market. This means there is anopportunity for advancing the field and revealing thebenefits of life logging through research and commercialefforts toward building and better understanding the valueof life-logs.Recent advances in sensor networks, pervasive comput-ing, communications and storage technologies enable us tosense and collect information about our life events digitally.The process of logging individuals’ experiences is notlimited to personal information: it can also be extended torecording community experiences and their online activi-ties. It can be expected that in the near future, life loggingsystems and electronic memories are going to have signif-icant impacts on our lives, perhaps even similar to therevolution brought by the Internet and mobile phones.Digital or electronic memories are records built up as aresult of life logging processes. In a more technical sense,the data set of life logging is referred to as e-memory.Based on Kro¨ner et al.’s [7] suggestion, here we categorizedigital memories into personal, community and objectmemory [14]. The personal memory is the result of lifelogging that targets a single user, while the communitymemory is the result of life logging that collects and cre-ates a digital memory from a group of users. In otherwords, one can refer to community memory as a collectionof personal memories, yet community memory is mostlyconcentrated on one or a few data sources, such as onlytwitter posts. However, the community memory is notnecessarily restricted to a community behavior, but it couldbe used to reveal personal information about each person inthat target community. There are no specific bordersbetween the personal, community and object memorycategories, and sometime research endeavors can fall intomore than one category.