The endoplasmic reticulum (ER) is a large, membrane-bound organelle important for protein, lipid, and steroid biosynthesis, carbohydrate metabolism, and calcium storage. Emanating from the nuclear envelope, this network of interconnected tubules forms a continuous sheet within the cytosol, enclosing a single internal lumen. Rough and smooth ER domains are identifiable through antibody-based detection of localised protein subsets; these are considered Endoplasmic Reticulum (ER) markers. The rough ER is decorated with ribosomes and hosts the synthesis of transmembrane proteins destined for organelle and plasma membranes, as well as water-soluble proteins targeted for secretion and organelle lumina. Polypeptide synthesis is initiated in the cytosol before an N-terminal ER signal sequence facilitates localisation of the emerging protein to the ER outer membrane. Next, co-translational protein import into the ER lumen is facilitated by the Sec61 translocon complex. Inside the lumen, nascent proteins are subject to processing activities including chaperone-mediated folding/refolding, protein glycosylation, and disulphide bond formation catalysed by the protein disulfide isomerase (PDI) enzyme. A heat shock protein family-member, BiP, prevents the mis-folding and aggregation of proteins by binding exposed hydrophobic regions. Other chaperone proteins, calnexin and calreticulin, specifically mediate folding of N-linked glycoproteins. Iterative folding efforts are regulated by quality control mechanisms which identify and eliminate proteins which are irrecoverably misfolded. The ER-associated degradation pathway (ERAD) promotes retro-translocation of misfolded proteins, returning them to the cytosol for proteasomal degradation. Activation of the ERAD forms part of a broader unfolded protein response (UPR); a surveillance mechanism required to maintain ER homeostasis. Initially, the UPR combats ER stress by increasing expression of critical chaperones and ERAD factors such as ERp29; however, severe overloading of misfolded proteins eventually necessitates programmed cell death by apoptosis. The smooth ER lacks ribosomes but instead has a membrane embedded with enzymes required for lipid synthesis. Primarily these are required to form phospholipids and cholesterol for cell membranes. Cells which specialise in lipid metabolism typically contain expanded smooth ER for large-scale cholesterol production as a precursor for steroid synthesis. Other roles of the smooth ER include the detoxification of toxic metabolites, carbohydrate metabolism, and calcium sequestration. The latter is facilitated by the SERCA ATPase pump which actively transports Ca2+ into the ER lumen from the cytosol. This Ca2+ reservoir is rapidly released to generate intracellular signals with high temporal specificity. Protein and lipid trafficking towards the Golgi apparatus is completed through vesicular transport from specific ER exit sites within the transitional ER; an intermediate domain lightly studded with ribosomes and therefore considered part rough, part smooth. In many cell types the demands for lipid synthesis can be met by the transitional ER alone, removing the requirement for a substantial smooth region. The ER is a dynamic, compartmentalised structure central to the metabolism of major biological polymers. ER markers are an important tool for understanding the processes within this organelle and the pathological consequences of their dysfunction in cancer and neurodegeneration. We offer a range of tracker dyes and ER marker antibodies, including Calreticulin antibodies, EGFR antibodies, GRP94 antibodies, and p53 antibodies, validated across multiple applications, and covering various host species, isotypes and conjugates.