This monograph gives a scholarly overview of the Swedish Evangelical Mission (SEM) in Ethiopia 1898-2015. It is a narrative with interpretations and analyses written as a reminder of the longstanding relations between Ethiopian Evangelical Christians and the SEM. The diachronic study highlights how Ethiopian and “Eritrean” Christians supported by SEM missionaries in Eritrea reached the Oromo people in western Wollega. The narrative shows how the mission was developing in Wollega in the periphery from 1898 and in Addis Ababa in the center of Ethiopia from 1904 by Ethiopian, “Eritrean” and Swedish pioneers. The SEM adopted a holistic model of mission with church, health care and school as three “constants” based in a mission station. The SEM A-plan was to reach the Oromos. The SEM B-plan was to reach other ethnic groups. The SEM C-plan was to renew the Ethiopian Orthodox Church. New pioneers were important actors in the developing evangelical work from 1920. The Italian occupation of Ethiopia 1936-1941 was an external factor that threatened the relations. The SEM returned in 1943 and a period of establishment started. In the early 1950s, the SEM work was evaluated by the missionaries to match the challenges of time and 1959 the Ethiopian Evangelical Church Mekane Yesus (EECMY) was founded. The SEM was integrated in the EECMY in 1969, which made the EECMY more independent as a church. The bright looking future was again threatened by external factors in the revolution period 1974-1991. The SEM supported Ethiopian evangelicals, who were threatened by persecution and imprisonment. A flexible mission strategy was now embraced, which took care of the opportunities at hand. The EECMY continued to grow, and in the post-revolution period 1992-2015 it evolved as the largest Lutheran church in the world. The SEM celebrated 150-years jubilee in Ethiopia and Eritrea (1866-2016) in Addis Ababa in 2015. The number of SEM missionaries had then decreased. The study contends that the longstanding relations between Ethiopian evangelical Christians and the SEM were crucial factors in the development of the EECMY. Christians were influenced to transcend barriers of ethnicity, social status and denominationalism. The multi-ethnic legacy, already from the small Bethel congregation in Massawa, continued in the EECMY and had an impact on Eritreans, Oromos and other ethnic groups. The cooperation of “the EECMY two Emmanuels”, Emmanuel Gebre Selassie and Emmanuel Abraham, is a case in point. As such, the study has relevance for today’s Ethiopia and for a wider African context. The study builds on material from written unpublished sources and written published sources in Swedish from EFS (SEM) historical archives in Uppsala, Sweden. They are supplied by interviews and more than hundred photos from EFS historical archives.