Archaeal viruses represent one of the least known territory of the viral universe and even less is known about their lipids. Based on the current knowledge, however, it seems that, as in other viruses, archaeal viral lipids are mostly incorporated into membranes that reside either as outer envelopes or membranes inside an icosahedral capsid. Mechanisms for the membrane acquisition seem to be similar to those of viruses infecting other host organisms. There are indications that also some proteins of archaeal viruses are lipid modified. Further studies on the characterization of lipids in archaeal viruses as well as on their role in virion assembly and infectivity require not only highly purified viral material but also, for example, constant evaluation of the adaptability of emerging technologies for their analysis. Biological membranes contain proteins and membranes of archaeal viruses are not an exception. Archaeal viruses as relatively simple systems can be used as excellent tools for studying the lipid protein interactions in archaeal membranes. 1. Introduction Viruses are obligate parasites. Their hallmark is the virion, an infectious particle made of proteins and encapsidating the viral genome. Many viruses, however, also contain lipids as essential components of the virion [1]. The majority of viral lipids are found in membranes, but viral proteins can also be modified with lipids [2, 3]. 1.1. Membrane Containing Viruses in the Viral Universe Membrane containing viruses can roughly be divided into two subclasses [1]. The first subclass contains viruses in which the membrane, also called an envelope, is the outermost layer of the viral particle. In the second class of viruses, the membrane is underneath the usually icosahedral protein capsid. Few viruses contain both the inner membrane as well as an envelope [1]. Lipid membranes of viruses have evolved into essential components of virions that in many cases seem to be involved in the initial stages of infection [4–6]. The majority of membrane containing viruses infect animals both vertebrate and invertebrate that do not have a cell wall surrounding the cytoplasmic membrane. For other host organisms such as plants and prokaryotes there are much fewer membrane containing viruses known [1]. Usually the cells of these organisms are covered with a cell wall. By far the majority of known viruses that infect prokaryotes, that is, bacteria (bacteriophages), and archaea (archaeal viruses) belong to the order Caudovirales, the tailed viruses (Figure 1) [1, 7]. These viruses are made of the icosahedrally
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