Microbes are ubiquitously found across plant surfaces and even within their cells, forming the plant microbiome. Many of these microbes contribute to the functioning of the host and consequently affect its fitness. Therefore, in many contexts, including microbiome effects enables a better understanding of the phenotype of the plant rather than considering the genome alone. Changes in the microbiome composition are also associated with changes in the functioning of the host, and there has been considerable focus on how environmental variables regulate plant microbiomes. More recently, studies suggest that the host genome also preconditions the microbiome to the environment of the plant, and the microbiome is therefore subject to evolutionary forces. Here, we outline how plant microbiomes are governed by both environmental variables and evolutionary processes and how they can regulate plant health together.