## 6 Conclusions  
LML delivery services have been a critical part of the supply chain management. Our findings in this study further indicate that additional research is required to enhance environmental, operational, and technical sustainability of LML delivery services. In this paper, we identified four new opportunities for LML: (i) Advanced LML services with new delivery techniques, (ii) innovative LML applications using new technologies and systems for environmental sustainability, (iii) effective management of uncertainties in LML, and (iv) LML for decentralized manufacturing services. The identified opportunities indicate that additional efforts in the LML research field should be given to enhance the operational, technological, and environmental sustainability of LML services.  
This paper presents a literature review of LML. The main goal of this review study was to provide a systematic review of LML-related studies by investigating the definition and the typology of LML, exploring issues and concerns in LML, and discussing new opportunities and future directions for the next generations of LML. In this literature review, more than 400 works written in English were initially identified, and 169 target research articles were eventually found to meet the study criteria and identified as complete LML studies that contributed to recent LML advances. Furthermore, to better understand the underlying themes addressed in the target research, text mining analysis was performed.  
Considering all reference papers comprehensively, a novel definition of LML and a typology of LML-related research are proposed in this paper. The challenges constraining the innovation of LML services are discussed with various current issues: (i) the development of information and communications technology has recently accelerated the introduction of the sharing economy era; (ii) using proximity stations or proximity points is a new way to improve the efficiency of LML, and the hub location problem has also has attention re-paid to consolidate last mile deliveries across urban areas to reduce traffic and cut greenhouse gas emissions; (iii) the innovation of techniques in LML services may be helpful to design environmentally sustainable LML; and (iv) various advanced delivery technologies can be contemplated (e.g., UAVs or drones, robots, CAVs, and EVs) to improve performance in LML services.  
To improve LML services, more efforts and suggestions should be addressed for various applications that are associated with new transportation modes for LML operations and their environmental sustainability, digital transformation, and novel methodology to improve LML operations of real industries, managing uncertainties in LML, and LML for decentralized manufacturing services. In this paper, such major opportunities for design and development of innovative LML services were discussed from technological and operational perspectives.  
We believe that this systematic literature review can serve as a useful tool for LML decision makers and stakeholders to model an efficient delivery system. In addition, the future directions and suggestions proposed in this paper can be leveraged for smart city designs with smart logistics systems considering sudden increases in last mile deliveries. Our study would leverage interdisciplinary collaboration with various logistics researchers and urban planners to develop a system to meet their needs.  
Conceptualization, H.S.N.; methodology, H.S.N.; software, K.P.; investigation, H.S.N., S.J.K., and K.P.; writing--original draft preparation, H.S.N., S.J.K., and K.P.; writing--review and editing, H.S.N., S.J.K., and K.P.; visualization, K.P.; and funding acquisition, K.P. All authors have read and agreed to the published version of the manuscript.  
This work was supported by Incheon National University Research Grant in 2020 for Kijung Park.  
The authors declare no conflict of interest.