Enveloped viruses like human immunodeficiency virus type 1 (HIV-1) enter cells by fusing their membrane with that of the cell plasma membrane, while others, like influenzavirus and alphavirus enter via uptake into endosomes and fusion with the endosomal membrane. For this they carry spike proteins. The spikes are composed of three copies of a subunit pair. One subunit has membrane fusion activity, whereas the other one binds to a structure (receptor) present on the cell surface. It also controls (chaperones) the activation of the fusion active subunit so that fusion does not occur prematurely. The fusion subunit is anchored in the virus membrane at one end and carries a fusion peptide (fp) at the other one. While hidden in the native spike the fp becomes exposed after spike activation (trigger). The fusion subunit interacts via the fp with the target membrane in an extended conformation and then forces the viral and the cell membranes together for fusion by a backfolding reaction. This model is mostly based on biochemical and structural studies using isolated subunits. However, a full understanding of the spike activation mechanism can only be obtained by studying the complete spike, i.e the trimer of the subunit pair. This was the aim of my thesis work. I used low dose electron cryomicroscopy (cryo-EM) to capture images of my objects that had been frozen in liquid ethane in their native and partly activated states. This procedure facilitates the analysis of the virus and the spikes in their intact form, free from e.g. staining artifacts. By computer aided processing of the particle images, the three-dimensional structures were obtained. For my studies I used an alphavirus, Semliki Forest virus (SFV), and two retroviruses, Moloney mouse leukemia virus (Mo-MLV) and HIV-1. The SFV spike is triggered by low pH in the cell endosome and the retrovirus spikes by receptor binding. One dilemma for SFV is that its spike during biosynthesis has to pass acidic compartments. How can it avoid activation? The chaperone and ...