Despite official regulation and in some cases repression, Protestant Christianity is one of the fastest growing religions in Mainland China today. In 1949, at the time of the Communist revolution, there were only 623,000 Protestant Christians in the mission churches in China, and they made up less than one-tenth of one per cent of the entire population. (1) Today, there are at least 50 million Protestants in China, and some scholars estimate that there are as many as 100 million. (2) Roughly 20 per cent of China's Protestants worship in churches affiliated with the government's Three-Self Patriotic Movement (Zhongguo jidujiao sanzi aiguo yundong weiyuan hui or sanzi jiaohui; hereafter, TSPM). (3) The rest worship in churches that are not recognised as official Protestant groups related to the TSPM. (4) This article draws on unique and valuable new data, as well as other primary and secondary sources, to examine the challenges and open spaces for non-TSPM Protestant groups in China's post-Mao period. It finds that four important factors influence the ability of non-TSPM Protestant leaders and groups to thrive within China's post-Mao political environment: (i) the behaviour of Protestant groups in the face of official rules and restrictions; (ii) the geographic location of Protestant groups; (iii) the extent and type of their personal connections (guanxi) with Party-state authorities; and (iv) political and material pressures on the local authorities with whom Protestants interact. Overall, the data and findings presented herein offer new insights into the complexity of Protestant church-state relations in China today. DATA SOURCES Supplementing the rich data that can be found in the existing scholarly literature on Mainland Chinese Protestant groups, (5) this article taps an array of additional sources. First, it consults reports prepared by human rights groups such as Human Rights Watch and the China Aid Association. Second, it examines primary documents produced by various levels of the Chinese Party-state, as well as primary documents and autobiographical accounts produced by members of Protestant churches. Third, it references non-academic studies by foreign journalists and observers who have extensive contacts with Mainland Chinese Protestant churches. (6) Fourth, it utilises information reported in nine roughly two-hour interviews conducted in 2009 and 2010 by the authors. The respondents included former Protestant church leaders and members currently residing in the United States who are still in contact with official and unofficial churches throughout China, and human rights lawyers who have defended the leaders of unofficial churches (see Appendix for a list of the questions posed to these interview subjects). Five of the nine respondents were in leadership positions in large unofficial Protestant churches or church networks when they were in China. At the time of the interviews, all of the respondents were active in Chinese Protestant churches in the US that minister to unofficial Protestant churches throughout China. Fifth, it draws on the personal experience of one of this article's co-authors, who was a member for 30 years (1978-2008) of an unofficial Protestant church that has been illegal in China since 1952. She was employed and later volunteered as a translator in the church's publishing companies in Taipei and the US from 1983 to 2001. Since 2002, she has helped nearly 100 Mainland Chinese members of her church and other unofficial churches to obtain asylum and settle in the US. She has also done freelance translation and interpretation work for the China Aid Association. Sixth, this article is based on the content of a random sample of 800 letters written by listeners in China to the Hong Kong-based Far East Broadcasting Company (hereafter FEBC) and its Voice of Friendship Seminary. The FEBC is a nondenominational, international Christian radio network that broadcasts the Good News in more than 130 languages from 128 transmitters located throughout the world. …