A basis of fundamental knowledge of different basic income approaches (BI) is essential to be elaborated for enhancing the prevalent debates on BI as an important solution on social transformation. Aiming to contribute to the current knowledge gaps and future research agendas on BI, this paper elaborates on BI theories and BI empirical cases, linkage, and gaps between these two parts. A total of 152 pieces of literature on BI theories and BI empirical cases were intensively reviewed, which includes BI definitions, positive and negative conceptual impacts of BI argued to bring to human society from social, economic, and policy-and-governance perspectives, as well as the implementation and the outcomes of 15 selected BI empirical cases. Our findings indicate that BI definitions remain imprecise, and the BI empirical cases are implemented in diverse ways. We also identified that many conceptual impacts of BI were not achieved in the reviewed BI empirical cases. We further argue that the policy environment for implementing BI has not been entirely created at present, and a key issue is that the ambiguity between the BI theories and BI empirical cases causes difficulties to generate widely agreed principles guiding the implementation of BI. This paper suggests that future studies on BI should focus more on revising existing BI definitions which need to be simplified, detailed, and unified, generating universal principles guiding the implementation of BI, and developing indicators measuring BI's consequences.